Around 140speciesof grasses naturallyoccurin thearidGreat Plains grasslands of North America. One key to theprairiegrasses' success is theirabilitytoconservewater in a dry environment. Like most plants, grasses take in waterthroughtheir roots and lose it as watervaporthroughtiny mouth-shaped valves, or stomata, in their leaves. The larger the surface of the leaf and the more stomata it bears, the greater the risk that the plant will lose too muchmoisturethroughevaporationandcollapse. Grasses are protected from thistraumaby having a reduced number of stomata and by the design of their leaves, which take the form of narrow blades. What's more, the surfaces of these reduced leaves are often modified - corrugated with ridges or covered in hairs - so that the wind can'tsweepacrossthe surface anddrawoutmoisture. The roughened surface holds a thinlayerofhumidairnext to the leaf and thus helps to reduce the "evaporativedemand," or drying power, of theatmosphere. Some grasses, including western wheatgrass, June grass, and blue grama,rollup the edges of their leaves during times ofdroughtto help keep their tissues from drying out.

Why aren't the stomata kept tightly closed tosealmoistureinside the leaf? Thereasonis that the stomata alsosupplyplants with freshairforphotosynthesis. If plants sealed their stomata, this life-sustainingprocesswould come to ahaltforlackofcarbondioxide. But if the stomata are thrown wide open, the plants risk deathdueto the loss ofmoisturethroughtheir gaping valves.

Prairiegrassesresolvethisdilemmabystrategicscheduling. In thefierceblazeof the midday sun, the stomata close so that watervaporis held in andcarbondioxideis kept out. In thisstate, the leaf cancapturesolarenergy and store it in energy-rich molecules (aprocessthat requires sunlight but notcarbondioxide). Then, in the cool of the evening, when theevaporativedemand drops off, the stomatasnapopen, letting watervaportrickle out but also permittingcarbondioxideto flood into the leaf. By mobilizing the energy that was stockpiled earlier in the day, the leaf uses thiscarbondioxidetomanufacturethe sugars and other molecules that it needs for growth (aprocessthat can beaccomplishedin total darkness). The result is thatprairiegrasses arepartiallynocturnal; they do most of their growing at night or in the early hours of the morning.

Prairiegrasses also have anotheringeniousway of evading the demands of the sun. Like many grassland creatures (prairiedogs, ground squirrels, cottontails, badgers, and so on), they take refugeunderground. What we think of as "grass"---the aboveground leaves and stems---actually constitutes less than half of theorganism. Between 60 and 80 percent of the plant, by weight, typically grows below ground. A 3-meter stand of big bluestem is anchoredundergroundby amassofcoarse,fibrousroots that reaches as much as 36 meters into the earth. Blue grama, for its part, seldom lifts its seed heads very far above the ground, but its network of fine, branching roots can sometimesprobethe soil for water almost two meters down.

Theseextensivesystems of roots push thirstilythroughthe soil,intenton sucking up everyavailabledropof water. But if the soil is very dry, as it is during periods ofdrought, the roots can'tdrawin enoughmoistureto keep pace with losses from the stomata. Grassesrespondby transferring their mostvaluableresources (including sugars and proteins) from their leaves into their roots and, especially, into their rhizomes (undergroundstems). Dead to the world above ground - withered and crisp - the plants live frugally below the surface, drawing on their cached supplies and biding their time until the weather improves. When the rainseventuallyreturn, as theyinevitablydo, the grassesexplodeintoaction, sending out fresh rhizomes, which in turn put out fresh leaves and roots, to produce aburgeoningnetwork of tender growth. The amazinglyresilientblue grama canrevivefromdormancy, become green, and grow on as little as five millimeters ofrainfall.

Questions:
question_stem: In paragraph 1, why does the author mention ridges and hairs?
choices: tag: A
text: To argue that these characteristics are not found on larger leaf surfaces

tag: B
text: To identify additional characteristics that help leaves conserve moisture

tag: C
text: To indicate the possible locations of stomata

tag: D
text: To provide examples of types of reduced leaves
answer_key: B
explanation: 答案 B。题干信息所在句子大意：leaves 的表面通常还有 ridges 或者 hairs, 这样风不会扫过叶子表面带走水分。也就是 ridges 和 hairs 可以阻止风带走叶子表面的水分。结合前文两句：The larger the surface…；Grasses are protected from…大意为叶子越大，气孔越多，植物失去水分的风险越大；Grass 可以通过 reduced number of stomata 以及 the design of their leaves 来降低这个失去水分的风险。由此可以认为题干所在的句子是另一种方法可以帮助植物保存水分。故而符合的答案为 B。

question_stem: What is the dilemma mentioned in the opening sentence of paragraph 3?
choices: tag: A
text: Prairie plants need to be close to each other, but the more plants there are in an area, the more difficult it is for each of them to get enough moisture for photosynthesis.

tag: B
text: Plants open the stomata to let in energy from the sun, but this sometimes leads to a buildup of too much carbon dioxide.

tag: C
text: When plants open their stomata, they lose water, but when they keep them closed, they cannot collect carbon dioxide for photosynthesis.

tag: D
text: In strong heat and sunlight, plants need to close their stomata, but this uses so much energy that the plants cannot grow strong.
answer_key: C
explanation: 答案 C。题干问本段开头句中提到的 dilemma 是什么。定位之后发现该句中存在指代关系“this dilemma”，所以需要结合前一段落进行理解。第二段中最后两句：If plants sealed their stomata…;But if the stomata are thrown wide open…大意为 stomata 关闭的话，carbon dioxide 缺乏，维持生命的过程会停止；stomata 打开的话，植物面临着因为失去水分而死亡的风险。所以本段第一句话的 this dilemma 即指的是打开或者关闭 stomata 会面临的问题，答案对应的为 C。

question_stem: In paragraph 3, why does the author include the phrase "can be accomplished in total darkness"?
choices: tag: A
text: To explain how prairie grasses are able to grow without losing a great deal of moisture

tag: B
text: To emphasize the damaging effects that sunlight can have on sugars and other molecules needed for growth

tag: C
text: To point out how even a little bit of light can interfere with the plant's ability to collect carbon dioxide

tag: D
text: To show that nocturnal growth is stimulated only after prairie grasses have collected moisture from the evening air
answer_key: A
explanation: 答案 A。题干所在句子大意：通过 mobilizing 白天早些时候储存的 energy，叶子用这些 carbon dioxide 就可以生产 sugars and other molecules that it needs for growth，这个过程可以完全在黑暗下进行。结合前文2句，In this state 意味着还需要往前看1句，从 In the fierce blaze…开始理解，大意为：正中午 stomata 闭合，水分闭合在叶子里，carbon dioxide 被隔绝在外。此时叶子 capture solar energy and store it in energy-rich molecules。然后，晚上，stomata 打开，水分和 carbon dioxide 可以自由进出，这时候在完全黑暗下原先储存的 energy 可以用来生产 sugars and other molecules that it needs for growth。这个过程也就解释了植物是如何在避免水分损失的情况下保持自身的生长发展。所以答案选择 A。B 中的 sugars and other molecules that it needs for growth 文章未提及；C 中的 a little bit of light can interfere with the plant's ability 与原文意思就不符合；D 中 prairie grasses have collected moisture from the evening air 不对，水分不是主要在夜间空气中收集的。

question_stem: According to paragraph 3, prairie grasses do each of the following during the night EXCEPT
choices: tag: A
text: collect and store energy

tag: B
text: manufacture sugars

tag: C
text: open the stomata

tag: D
text: gather carbon dioxide
answer_key: A
explanation: 答案 A。第2、3句中提到，in the midday, leaf can capture solar energy and store it in energy-rich molecules，所以 A 不是在晚上进行的。B 对应倒数第二句，accomplished in total darkness，符合夜间进行；CD 都在倒数第三句话中出现：in the cool of the evening…the stomata snap open… permitting carbon dioxide to flood into the leaf。

question_stem: According to paragraph 4, why do prairie dogs, ground squirrels, and other grassland creatures burrow underground?
choices: tag: A
text: To find grass roots to eat

tag: B
text: To hide from predators

tag: C
text: To look for water

tag: D
text: To avoid the sun
answer_key: D
explanation: 答案 D。根据题干信息 prairie dogs, ground squirrels、burrow underground 定位到段首，大意为：Prairie grasses 有独创的方式来 evading the demands of the sun。就像 prairie dogs, ground squirrels，它们也在地下寻求庇护。所以这些动物挖洞到地下的目的是为了躲避 sun，答案符合的为 D。

question_stem: Paragraph 5 suggests that when there is plenty of water available in the soil, grasses
choices: tag: A
text: lose many of their rhizomes

tag: B
text: store sugars and proteins in their leaves

tag: C
text: use as little energy as possible

tag: D
text: take in moisture mostly through the stomata
answer_key: B
explanation: 答案 B。根据题干信息 plenty of water available in the soil 定位到第二句，第二句中提到的是水分缺少的情况，与题干相反，因为推理题，所以可以根据文本信息反推，原文意思是 soil 水分缺少时，roots 无法吸收足够的水分来跟上 stomata 的失水的速度；Grasses 的反应就是将 sugars and proteins 等营养物质从 leaves 转移到 toots。所以如果 soil 中有充足的水分，情况则相反，营养物质大都集中在叶子部分，不会转移到 roots，所以答案选 B。AC 未提及；D 不对，文章提到吸水的主要是植物的 root，stomata 是水分流失的途径。

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: Until the weather improves, the plant must grow many new leaves to replace those that have died above the surface.

tag: B
text: When the weather changes suddenly, the parts of the plants that are not protected by the ground may wither and die from the shock.

tag: C
text: The plants still surviving underground will, when the weather improves, take the place of other plants that have died.

tag: D
text: Until the weather improves, plants that appear lifeless survive underground by using heir stored supplies economically.
answer_key: D
explanation: 答案 D。原句大意：地上的植物已经死去(枯萎并干脆)，地下的植物节约地生活着，利用存储的物资等待时机，直到天气好转。A 中的 the plant must grow many new leaves…原句未提到，无中生有；B weather changes suddenly 未提及，同时后半句意思也是无中生有；C 中 will take the place of other plants that have died 原句未提及，无中生有；D 大意为：看似无生机的植物在地下通过经济地利用其储存的物质生存下来，直到天气好转。所以大意匹配的只有 D。

question_stem: The word "resilient" in the passage is closest in meaning to
choices: tag: A
text: quick to recover

tag: B
text: brightly colored

tag: C
text: fast growing

tag: D
text: complex
answer_key: A
explanation: 答案 A。resilient：有弹力的，能复原的。A：快速恢复；B：色彩鲜艳的；C：快速增长；D：复杂的，合成的。