Organisms of differentspeciesfrequentlyevolveadaptations that make themutterlyandspecificallydependent on each other for resources. For example, many trees must have mycorrhizalfungiliving in theirrootsystems,fungithat areindispensableto the tree because theyfacilitatemineraluptake from the soil. In turn, thefungirelyon the trees to photosynthesize; they use some of the tree’s chemical food as their only energysource. Suchintimaterelationships are examples of coevolutionthroughwhich twospeciesbecome so interdependent that they canthriveonly in each other’s presence.

At firstglance, the relationship between pinyonpinetrees and the pinyon jay does notappearto be one of coevolution. Pinyon pines produce cones and seeds thatattractseed predators, especially the pinyon jay. A seed, ofcourse, contains anembryonicplant for the nextgeneration. Why sacrifice it to an animal? Some plants havepoisonousseeds, anobviousadaptationto reduce loss to animals. Pinyons, however, have anarrayofcharacteristicsthatcombinetoencouragejays to visit the pines and help themselves to the seeds. Cones are positioned upward and outward on the tree, so the seeds inside are inplainsight of the jays,essentiallyinvitingthem to partake. Pinyon seeds are unusually large, and each seed is high in energy. The seed coat is thin, meaning that birds such as pinyon jays can not onlyingestthe seeds but alsodigestthem. In many plants, an indigestible seed coat permits the seed to pass unharmedthroughthe bird’s alimentary system. Pinyon seed coats differ in color betweenedibleand nonviable seeds, signaling the jays as to which they should select.

Attracting seed predators would notseemto be a successfulsurvivalstrategyfor pinyon pines. However, pinyon jays are behaviorally adapted to bury any seeds inexcessof theirimmediatesurvivalneeds. This is a usefulbehaviorfor the jays, providing they canretrievesome of the buried seeds during winter, and good for the trees, as the unretrieved seeds are ready togerminate. Still, for the jays to bury any seeds, there must be anabundanceof seeds far beyond the jays’immediateneeds. One tree could never produce so many seeds, but if all the pinyons in aregionproduced heavy seed crops at once, they would indeed “flood themarket” with vastly more pinyon seeds than thelocalpopulationof jays couldconsume. In fact, that is exactly what the pinyon pines do.

It requires a greatdealof energy to make so many seeds, so much energy that it isunlikelythat a pinyonpopulationcould produce such a bumper seed crop every year. More important, however, it would be to theseveredisadvantageof the trees to produce large seed crops annually,evenif they could. Doing so would make theresourcenot onlyabundantbut alsopredictable. Seed predators could, over the years, steadily build their populations,eventuallyincreasing so much that they could, indeed,consumevirtuallyall of the seeds. It is much moreadaptivefor the plants to produce seed cornucopiasintermittently. Doing so has severalmajoradvantages. First, energy can be stored for some years and then devoted to cone and seed production, ensuringadequateenergy to produce a large seed crop. Second, seed-predator populations willdeclinein years of low seed production, eitherthroughstarvation, reducedreproduction, oremigration. Pinyons in most areas have aroughlysix-yearintervalbetween heavy seed crops.

Pinyon jays have so completely adapted to thecycleof the pinyons that theirreproductionis tied to it. Mostspeciesof birdsmaturesexually inresponseto changes in day length. In pinyon jays, however, day length is only onestimulusforreproduction. The othercueisavailabilityof pinyon seeds. When seeds areabundant, jays canbreedvery early inspring, continuebreedingthroughsummer, and reenterbreedingcondition as early as the winter solstice. So the jaypopulationtemporarilyenlarges inresponseto one of the intermittent large crops of pinyon seeds. But the increase in thepopulationof jays is less than the increase in the size of the seed crop. Thus many of the seeds of a bumper crop are buried andsubsequentlygerminate, many more than wouldgerminatewithout planting by the jays.

Questions:
question_stem: The word “indispensable” in the passage is closest in meaning to
choices: tag: A
text: useful

tag: B
text: necessary

tag: C
text: available

tag: D
text: adapted
answer_key: B
explanation: 所在句为例子，为前面观点做解释。前面观点是：不同物种的有机体经常进化出适应能力，使它们在资源上完全和具体地相互依赖。那么例子中 fungi 和 tree 是彼此依赖。  A.有用 B.必要 C.可用 D.改编，所以选 A。

question_stem: According to paragraph 1, mycorrhizal fungi need to live in root systems of trees in order to
choices: tag: A
text: absorb minerals

tag: B
text: gain energy

tag: C
text: help the trees photosynthesize

tag: D
text: evolve
answer_key: B
explanation: 按定位词 mycorrhizal fungi   root systems of trees 找到 In turn, the fungi rely on the trees to photosynthesize; they use some of the tree’s chemical food as their only energy source. 反过来，真菌依赖树木进行光合作用；它们利用树木的一些化学食物作为唯一的能源。如果要区分其他选项 要把这个队里关系中的代词弄清楚。 For example, many trees must have mycorrhizal fungi living in their（trees） root systems, fungi that are indispensable to the tree because they (fungi) facilitate (是促进 root 获得)mineral uptake from the soil. In turn, the fungi rely on the trees to photosynthesize; they（fungi） use some of the tree’s chemical food as their (fungi)only energy source.  所以：A.吸收矿物质的是树根 C.进行光合作用的是真菌 D.进化文中没提，所以选 B。

question_stem: The word “partake” in the passage is closest in meaning to
choices: tag: A
text: discover

tag: B
text: see

tag: C
text: remove

tag: D
text: consume
answer_key: D
explanation: 前一句句间转折后给出主要观点：Pinyons, however, have an array of characteristics that combine to encourage jays to visit the pines and help themselves to the seeds. 然而，松树有一系列的特性，这些特性结合在一起，可以鼓励松树参观松树并帮助自己获取种子。 Cones are positioned upward and outward on the tree, so the seeds inside are in plain sight of the jays 这些是特性 那么后面内容就是邀请鸟获取/吃种子 A.发现 B.看见 C.移除 D.消费/吃掉，所以选 D。

question_stem: According to paragraph 2, all the following are characteristics of pinyon pine seeds that make them attractive to seed predators EXCEPT:
choices: tag: A
text: The seeds are highly visible.

tag: B
text: The seeds contain a large amount of energy.

tag: C
text: The seeds pass unharmed through the predator’s alimentary system.

tag: D
text: The seed coat is thin.
answer_key: C
explanation: 按 characteristics，pine seeds，attractive to seed predators 定位，并且在3题之后，所以 so the seeds inside are in plain sight of the jays, essentially inviting them to partake. Pinyon seeds are unusually large, and each seed is high in energy. The seed coat is thin, meaning that birds such as pinyon jays can not only ingest the seeds but also digest them. 所以里面的种子在鸟的视线里很明显=A，基本上是邀请它们分享。松子种子异常大，每颗种子的能量都很高=B。种皮很薄=D，这意味着像松鸦这样的鸟类不仅可以吞食种子，还可以消化它们。C 种子通过捕食者的消化系统而安然无恙。原文没提种子是否 unharmed; 后面的句子 In many plants, an indigestible seed coat permits the seed to pass unharmed through the bird’s alimentary system. 在许多植物中，不易消化的种皮使种子能够安然无恙地通过鸟类的消化系统，提到的是其他不易消化的植物，不是 pine，所以选 C。

question_stem: What purpose does paragraph 2 serve in the passage as a whole?
choices: tag: A
text: It presents an exception to the ideas about coevolution provided in paragraph 1.

tag: B
text: It presents a puzzle about pinyon pines that later paragraphs will resolve.

tag: C
text: It introduces for further discussion the fact that pinyon pines do not produce poisonous seeds.

tag: D
text: It formulates a hypothesis that the rest of the passage will support about why pinyon pines attract seed predators.
answer_key: B
explanation: 问段落在全篇的作用，优先处理本段段意 再处理其他段段意（全文中心思想） Pinyon pines produce cones and seeds that attract seed predators, especially the pinyon jay. A seed, of course, contains an embryonic plant for the next generation. Why sacrifice it to an animal? 松树的球果和种子能吸引捕食者，尤其是松树。当然，种子中含有下一代的胚胎植物。为什么要把它献给动物？=puzzle 下一段段意讲：吸引种子捕食者似乎不是松树成功的生存策略，再下一段：good for the trees, as the unretrieved seeds are ready to germinate 对树木有好处 利于种子发芽…所以选 B。    A.第1段中关于共同进化的思想的一个例外，2段首句，乍一看，松树和松鸦之间的关系似乎不是共同进化的关系，欲扬先抑，为了引出后文 Pinyons, however, have an array of characteristics that combine to encourage jays to visit the pines and help themselves to the seeds. 首句不是段意，转折是段意。转折否定前者，给出后者恰恰是共同进化关系，A 错；C 中松树不产生有毒种子这一事实是为了加强 puzzle，后文没有进一步讨论为什么松树不产生有毒种子。 D 它提出了一个假设，即文章的其余部分将支持为什么松树会吸引种子捕食者，全文主旨是二者相互受益，其余部分并不是只有松树吸引种子捕食者。

question_stem: It can be inferred from paragraph 4 that the reason the pinyon jay population in an area declines for several years in succession is that
choices: tag: A
text: after several years during which pinyon trees produce no seeds, there are not enough young pinyon trees producing seeds to feed the jays

tag: B
text: for several years after a heavy seed crop, pinyon pines stop producing enough seeds to support a large population of jays

tag: C
text: for several years in succession, pinyon jays spend most of their energy in collecting and burying seeds rather than in reproducing

tag: D
text: over a period of several years, the pinyon jays in an area are displaced by other seed-predator species
answer_key: B
explanation: 按 pinyon jay population  declines  several years in succession 定位，找到 Second, seed-predator populations will decline in years of low seed production, either through starvation, reduced reproduction, or emigration. 第二，在种子产量低的年份，种子掠食者的数量将减少，这可能是由于饥饿、繁殖减少或移民。 选项没有饥饿、繁殖减少或移民，显然我们要找到饥饿、繁殖减少等的根本原因。 It requires a great deal of energy to make so many seeds, so much energy that it is unlikely that a pinyon population could produce such a bumper seed crop every year.  It requires a great deal of energy to make so many seeds, so much energy that it is unlikely that a pinyon population could produce such a bumper seed crop every year.  产种子需要大量能量，松树不能每年高产，能量可以储存几年，然后用于球果和种子生产，确保有足够的能量来生产大量的种子作物，所以 pinyon jay population 减少应该是松树储存能力等待大量生产的间隔期 注意：间隔期不是松树不生产种子，是松树不大量生产种子。所以 A 的 produce no seeds 错， B 正确， C、D 原文没有提到。

question_stem: According to paragraph 4, which of the following is a reason why pinyon pines do not produce a bumper seed crop every year?
choices: tag: A
text: Pinyon jays would stop burying pinyon seeds if a bumper crop of the seeds was produced every year.

tag: B
text: Pinyon pines are genetically programmed to produce annual seed crops of varying sizes.

tag: C
text: With any fixed level of seed production, populations of seed predators would eventually increase to consume the available crop of seeds.

tag: D
text: The environment cannot support as many pinyon pines as a yearly bumper seed crop would produce.
answer_key: C
explanation: 按 pinyon pines do not produce a bumper seed crop every year 定位到原文 It requires a great deal of energy to make so many seeds, so much energy that it is unlikely that a pinyon population could produce such a bumper seed crop every year. More important, however, it would be… predictable. Seed predators could, over the years, steadily build their populations, eventually increasing so much that they could, indeed, consume virtually all of the seeds. It is much more adaptive for the plants to produce seed cornucopias intermittently.   制造这么多种子需要大量的能量… 不太可能每年生产出如此丰收的种子作物。然而，如果（这句是虚拟语气 假的）每年都能种出更多的种子，那就更重要了。这样做不仅可以使资源丰富而且可以预测。多年来，种子掠食者可以稳定地繁殖它们的种群，最终数量增长得如此之大，以至于它们可以吃掉几乎所有的种子。所以，植物产生适应性 断断续续产生种子。所以不能每年大量产种子。1是消耗太大能力；2是生产再多，掠食者也会不断增长，最终都会吃掉。选项 C 提到的是第二点，所以选 C，其他没提到。

question_stem: What purpose does the discussion of paragraph 5 serve in the passage as a whole?
choices: tag: A
text: It shows that the population size of pinyon jays depends on pinyon pines but that the population size of pinyon pines does not depend on pinyon jays.

tag: B
text: It provides evidence that pinyon trees benefit more from pinyon jays than the jays benefit from the pines.

tag: C
text: It establishes that, fully understood, the relationship between pinyon pines and pinyon jays is one of coevolution.

tag: D
text: It explains why in years of bumper seed crops, pinyon jays are unable to increase their population to match the increase in the number of available pinyon seeds.
answer_key: C
explanation: 如5题，问段落在全篇的作用，优先处理本段段意，再处理全文中心思想。通常段落都是为中心思想服务的，要么支持中心思想，要么是中心思想的反例。本段段意（首句）松鸦鸟完全适应了松树的循环，因此它们的繁殖与之息息相关它确立了松树和松鸦之间的关系是一种共同进化（全文中心思想）的关系 所以选 C。