A symbiotic relationship is aninteractionbetween two or morespeciesin which onespecieslives in or on anotherspecies. There are three main types of symbiotic relationships: parasitism, commensalism, andmutualism. The first and the third can be key factors in the structure of a biologicalcommunity; that is, all the populations of organisms living together and potentially interacting in aparticulararea.

Parasitism is a kind of predator-prey relationship in which oneorganism, theparasite, derives its food at the expense of its symbioticassociate, thehost. Parasites are usually smaller than their hosts. An example of aparasiteis a tapeworm that lives inside the intestines of a larger animal and absorbs nutrients from itshost. Naturalselectionfavors the parasites that are best able to find and feed on hosts. At the same time,defensiveabilities of hosts are also selected for. As an example, plants make chemicalstoxictofungalandbacterialparasites,alongwith onestoxictopredatoryanimals (sometimes they are the same chemicals). In vertebrates, theimmunesystem provides amultipledefenseagainstinternalparasites.

At times, it is actually possible to watch the effects of naturalselectionin host-parasite relationships.

For example, Australia during the 1940 s was overrun by hundreds of

millions of European rabbits. The rabbits destroyed huge expanses of Australia and threatened the sheep andcattleindustries. In 1950, myxomavirus, aparasitethat affects rabbits,

wasdeliberatelyintroduced into Australia to control the rabbitpopulation. Spreadrapidly by mosquitoes, thevirusdevastated the rabbitpopulation. Theviruswas lessdeadlyto theoffspringof surviving rabbits, however, and it caused less and less harm over the years. Apparently, genotypes (thegeneticmake-up of anorganism)

in the rabbitpopulationwere selected that were better able toresisttheparasite. Meanwhile, the deadliest strains of thevirusperished with their hosts as naturalselectionfavoredstrains that could infect hosts but not kill them. Thus, naturalselectionstabilized this host-parasite relationship.

Incontrastto parasitism, in commensalism, one partner benefits withoutsignificantlyaffecting the other. Few cases ofabsolutecommensalism probably exist, because it isunlikelythat one of the partners will be completely unaffected. Commensalassociations sometimes involve onespecies' obtaining food that isinadvertentlyexposedby another. Forinstance, several kinds of birds feed on insects flushed out of the grass bygrazingcattle. It is difficult to imagine how this couldaffectthecattle, but the relationship may help orhinderthem in some way not yet recognized.

The third type of symbiosis,mutualism, benefits both partners in the relationship Legume plants and their nitrogen-fixingbacteria, and the interactions between flowering plants and their pollinators, are examples ofmutualisticassociation. In the first case, the plants provide thebacteriawith carbohydrates and otherorganiccompounds, and thebacteriahave enzymes that act as catalysts thateventuallyaddnitrogento the soil, enriching it. In thesecondcase, pollinators (insects, birds)obtainfood from the flowering plant, and the plant has itspollendistributed and seeds dispersed much more efficiently than they would be if they were carried by the wind only. Another example ofmutualismwould be the bull'shornacacia tree, which grows in Central and South America. The tree provides a place to live for ants of the genus Pseudomyrmex. The ants live in large,hollowthorns and eat sugar secreted by the tree. The ants also eat yellow structures at the tip of leaflets: these areproteinrich andseemto have nofunctionfor the tree except toattractants. The antsbenefitthehosttree by attackingvirtuallyanything that touches it. Theystingother insects and large herbivores (animals that eat only plants) andevenclip surroundingvegetationthat grows near the tree. When the ants are removed, the trees usually die, probably because herbivores damage them so much that they are unable tocompetewith surroundingvegetationfor light and growingspace.

Thecomplexinterplayofspeciesin symbiotic relationships highlights an important point about communities: Their structure depends on a web ofdiverseconnections among organisms.

Questions:
question_stem: Which of the following statements about commensalism can be inferred from paragraph 1?
choices: tag: A
text: It excludes interactions between more than two species.

tag: B
text: It makes it less likely for species within a community to survive.

tag: C
text: Its significance to the organization of biological communities is small.

tag: D
text: Its role in the structure of biological populations is a disruptive one.
answer_key: C
explanation: C 以 commensalism 做关键词定位至第二句，说有三种类型，第一种和第三种很重要，根据对比，也就是说第二种 commensalism 不重要，所以答案是 C。

question_stem: The word derives in the passage is closest in meaning to
choices: tag: A
text: Digests

tag: B
text: Obtains

tag: C
text: Controls

tag: D
text: Discovers
answer_key: B
explanation: B derive: 起源于，得自。A 是消化；B 是获得；C 是控制；D 是发现。所以正确答案是 B 的 obtain。

question_stem: According to paragraph 2. which of the following is true of the action of natural selection on hosts and parasites?
choices: tag: A
text: Hosts benefit more from natural selection than parasites do.

tag: B
text: Both aggression in predators and defensive capacities in hosts are favored for species survival.

tag: C
text: The ability to make toxic chemicals enables a parasite to find and isolate its host.

tag: D
text: Larger size equips a parasite to prey on smaller host organisms.
answer_key: B
explanation: 以 natural selection 做关键词定位至第三四句： Natural selection favors the parasites that are best able to find and feed on hosts. At the same time, defensive abilities of hosts are also selected for. 说自然选择会选择那些最容易寻找寄主的寄生者，同时也会选择寄主的防御能力，所以答案是 B。A 的比较，C 的 toxic material 和 isolate，还有 D 的大小均没有相关信息。

question_stem: The word devastated in the passage is closest in meaning to
choices: tag: A
text: Influenced

tag: B
text: Infected

tag: C
text: strengthened

tag: D
text: destroyed
answer_key: D
explanation: D devastate: 毁灭。A 是影响；B 是感染；C 是加强；D 是破坏。所以 D 的 destroy 正确。

question_stem: Which of the following can be concluded from the discussion in paragraph 3 about the Australian rabbit population?
choices: tag: A
text: Human intervention may alter the host, the parasite. and the relationship between them.

tag: B
text: The risks of introducing outside organisms into a biological community are not worth the benefits.

tag: C
text: Humans should not interfere in host-parasite relationships.

tag: D
text: Organisms that survive a parasitic attack do so in spite of the natural selection process.
answer_key: A
explanation: A 问题问的是兔子实验得出什么结论，先找到兔子实验，发现兔子实验就是人类干预自然选择的一个例子，但千万不要过于详细看实验的内容，因为问的是结论，所以关注首尾句。第一句说可以预见自然选择对于寄生关系的影响；而最后一句话说自然选择使寄生关系稳定，在结合例子中说的人类干预之后，通过 natural selection，最终通过人类干预的 parasite relationship 稳定下来了。所以答案是 A，人类可以改变寄生关系；原文没有将引入生物的优缺点比较，B 错；也没有讲作者对引进生物的态度，C 错；D 也没说

question_stem: According to paragraph 3, all of the following characterize the way natural selection stabilized the Australian rabbit population EXCEPT:
choices: tag: A
text: The most toxic viruses died with their hosts.

tag: B
text: The surviving rabbits were increasingly immune to the virus.

tag: C
text: The decline of the mosquito population caused the spread of the virus to decline.

tag: D
text: Rabbits with specific genetic make-ups were favored.
answer_key: C
explanation: C EXCEPT 题，排除法，A 的 most toxic viruses 做关键词定位至倒数第二句的 deadliest strains of the virus，所以 A 正确，不选；B 的 surviving rabbits 做关键词定位至倒数第四句，正确，不选；C 的 mosquito population 做关键词定位至第五句，跟选项说的完全无关，所以 C 错，选；D 的 genetic make-up 做关键词定位至倒数第三句，所以 D 正确，不选。

question_stem: The word inadvertently in the passage is closest in meaning to
choices: tag: A
text: Indefensibly

tag: B
text: Substantially

tag: C
text: Unintentionally

tag: D
text: Partially
answer_key: C
explanation: C inadvertently: 非故意的，不注意地。A 是不能防御地；B 是实质上地，大体上地；C 是非故意地；D 是部分地，偏袒地。所以正确答案是 C 的 unintentionally。

question_stem: According to paragraph 5, the relationship between legumes and bacteria benefits the soil by
choices: tag: A
text: adding enriching carbohydrates

tag: B
text: speeding the decay of organic matter

tag: C
text: destroying enzymes that pollute it

tag: D
text: contributing nitrogen to it
answer_key: D
explanation: D 以 legume，bacteria 和 soil 做关键词定位至第二句，说植物给细菌提供有机物，而细菌中的酶可以作为催化剂向土壤中添加氮，所以答案是 D，其他都没说。

question_stem: Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
choices: tag: A
text: The relationship between flowering plants and pollinators provides pollinators with food and flowers with efficient reproduction.

tag: B
text: In some cases birds obtain food from the seeds that are dispersed in the wind.

tag: C
text: The wind not only helps the flowers distribute their seeds but enables birds to find more food.

tag: D
text: Animals and insects are more effective in distributing pollen and seeds than the wind.
answer_key: A
explanation: A 原句的结构是 pollinator 和植物互惠互利，所以正确答案是 A。B 混淆概念，pollinator 吃的不是 seed；C 同样混淆概念，没 wind 的事儿；D 只是说植物从中得到的好处，没说 pollinator 的好处，缺失重要信息，错。

question_stem: According to paragraph 5. which of the following is NOT true of the relationship between the bull's horn acacia tree and the Pseudomyrmex ants?
choices: tag: A
text: Ants defend the host trees against the predatory actions of insects and animals.

tag: B
text: The acacia trees are a valuable source of nutrition for the ants.

tag: C
text: The ants enable the acacia tree to produce its own chemical defenses.

tag: D
text: The ants protect the acacia from having to compete with surrounding vegetation.
answer_key: C
explanation: C EXCEPT 题，排除法，A 的 predatory actions 和 insects and animals 做关键词定位至倒数第二句，所以 A 正确，不选；B 的 nutrition 做关键词定位至倒数第四句的 protein rich，正确，不选；C 的 chemical defenses 在原文没有对应点 C 错，选；D 的 surrounding vegetation 做关键词定位至最后一句，所以 D 正确，不选

question_stem: The word highlights in the passage is closest in meaning to
choices: tag: A
text: Defines

tag: B
text: Emphasizes

tag: C
text: Reflects

tag: D
text: Suggests
answer_key: B
explanation: B : highlight: 强调，突出。A 是定义；B 是强调；C 是反应；D 是暗示。所以 B 的 emphasize 正确。

question_stem: What is the main purpose of this passage?
choices: tag: A
text: To explain the concept of symbiosis by expanded descriptions of its principal types

tag: B
text: To make a comparison between human relationships and symbiotic interactions in the natural world

tag: C
text: To demonstrate the unforeseen benefits of natural processes that at first seem wholly destructive

tag: D
text: To argue that parasitism is a problem that can be solved by scientific intervention
answer_key: A
explanation: A，问的是整个文章的目的是什么，所以应该关注各段的中心句。文章第一段就说共生关系有多种类型，接着分几段分别说了几个类别的共生关系，所以答案是 A，分类描述了概念。B 错，不是比较自然和人；C 和 D 都不是全文所说。