In open grasslands there is no place for a large animal to hide. Thus a watchfulgrazinganimal will see theslightmovement that betrays the presence of apredatorlong before it is close enough tolaunchan attack. It sounds as though the hunters (predators) stand no chance at all. Unfortunately for the grazers, life is not so simple, however. Agrazinganimal must lower its head and look at the ground to feed. Its attention may be occupied for only a few seconds before it raises its head and resumes its watch while chewing the food it took, but hunters are patient and skillful and are concentrating intensely. Those few seconds provide time enough to advance a few steps and then freeze, body flattened against the ground. It may take hours, buteventuallythese repeated small advances will put the hunterwithinrange—close enough to outrun itsprey—and the long time the hunt has taken will have beenworthwhile, because the resulting feast will be highly nutritious.

Clearly the grazers are at a disadvantage, because while they eat they arevulnerableto attack. The hunters also have a weakness, however, and it is one that allows the grazers to survive. Hunters can attack only onepreyanimal at a time. This applies even to the predators that hunt as a team, such as lionesses, wolves, and hunting dogs. Their hunt involves running down or ambushing an individual. Teamwork allows them to hunt animals much bigger and stronger than themselves and to hunt more successfully, but it does not allow them to attack more than one individual at a time.

The grazersexploitthis weakness by making it as difficult as they can for the predators to choose an individual as a target. They do not graze alone,scatteredwidely across the landscape, but together, as aherd. The approaching hunter sees not asolitaryanimal, but a crowd of animals, all of them moving, so they areconstantlycrossing and recrossing each other’s paths. No sooner does the hunter choose an individual than another animal has crossed in front of it and the target has disappeared into theherd. From the hunter’s point of view this is highly confusing behavior—as, indeed, it is meant to be.

There is another advantage to the grazers: Aherdis much morealertthan asolitaryanimal. An animal has to relax its guard while it is taking food, but in aherdthere are at any time some animals with their heads down, biting, and others, with their heads up, watching. What is more, those with their heads up are looking in different directions so that together they arealertto any movement anywhere on the landscape around them. There is no way for a hunter toapproachaherdunobserved. When a member of theherdspots trouble, it starts to move away. Other members of theherdmove with it and the entireherdstarts to move. If the trouble is serious and close, theherdwill run. The individual raising thealarmissimplyprotecting itself, but in doing so it is warning all of the others.

Herding is highly successful, provided members of theherdstay together in a tightbunch. The hunter moves with theherd, watching for an individual towanderaway from the others. When that happens, it tries to move between that individual and the rest of theherd, preventing it from rejoining. Once it has done that the hunter has a good chance of making a kill. If theherdstarts to run, asolitaryhunter may abandon the chase, but apackof wolves or hunting dogs will regard the runningherdof animals as an opportunity and set off inpursuit. As theherdruns, one or two old or sick animals, or young animals that become separated from their mothers, may fall behind.

As soon as the hunter or hunters seize theirprey, they lose interest in all other grazers since then they, too, must concentrate on eating, at which point theherdstops running, those who were left behind rejoin the group, and they allresumegrazing.

Questions:
question_stem: According to paragraph 1, which of the following presents a serious difficulty for the predators that hunt animals grazing in grasslands?
choices: tag: A
text: It is difficult to find grazers in grasslands when they are feeding with their heads down.

tag: B
text: Predators lack places to hide from prey in grasslands.

tag: C
text: It is difficult for predators to outrun grazers on grasslands.

tag: D
text: Grazers in open grasslands tend to be very large animals.
answer_key: B
explanation: 题干问 predators 遇到的困难，定位句在开头前2句话：在开阔的大草原上，大型捕食者无处可藏，所以食草动物会随时察觉到敌人的行踪，跟 B 直接对应上。

question_stem: Which of the following can be inferred from paragraph 1 about grazing animals?
choices: tag: A
text: They spend significantly more time looking down at the ground than they do with their heads raised.

tag: B
text: They tend to remain within a relatively small area for extended periods of time.

tag: C
text: They are extremely difficult for predators to see as long as they remain motionless.

tag: D
text: They can run significantly faster than most of their predators.
answer_key: B
explanation: 根据段落第5句话到最后，可知：吃草动物每次低头吃草的那几秒钟里，无法看到周边的情况，捕食者会利用这几秒，偷偷前进几步，然后趴下不动。有时候会长达几个小时，但是最终会通过每次前进的几步，最终到达离吃草动物足够近的地方把食草动物抓住，由此可以退出，食草动物会通常一连好几个小时在一个相对小的片区吃草，对应答案 B。答案 A，比较级不存在，排除；C，食草动物和 predators 位置互换才是对的；D，根据文章最后一句破折号中的 outrun its prey 可知，predators 比食草动物跑得快，所以 D 错。

question_stem: According to paragraph 2, what is an advantage to predators that hunt as a team?
choices: tag: A
text: A team can kill animals that are much bigger and stronger than any of the predators could take on by itself.

tag: B
text: A team can ambush their prey without having to run them down as an individual predator has to do.

tag: C
text: A team can attack more prey animals at one time than an individual predator could.

tag: D
text: A team is less vulnerable to an aggressive response from grazers.
answer_key: A
explanation: 根据原文“ Teamwork allows them to hunt animals much bigger and stronger than themselves and to hunt more successfully”可知 A 正确。

question_stem: In paragraph 2, why does the author describe the hunting technique used by predators that hunt as a team?
choices: tag: A
text: To explain why predators that hunt as a team wait until grazers start eating to attack

tag: B
text: To argue that species that hunt as a team do not have the same weakness as individual predators

tag: C
text: To emphasize that even predators that hunt in groups cannot hunt and attack more than one prey at a time

tag: D
text: To help explain why grazers are at a particular disadvantage with certain predatory animals
answer_key: C
explanation: 目的题考核题干细节所要论证的观点句，该题目中，观点句是第3句话” Hunters can attack only one prey animal at a time.” 捕食者每次只能攻击一只食草动物。对应选项 C，及时是群体合作捕食，也只能捕食一直猎物，无法同时攻击多只猎物。

question_stem: Which of the following is NOT mentioned in paragraph 4 as a way in which animals that graze as a herd benefit?
choices: tag: A
text: Some animals in the herd will be in a position to observe danger while others have their heads down.

tag: B
text: Each animal in a herd tries to protect the others from danger.

tag: C
text: As a herd, animals can observe the landscape in a variety of directions at once.

tag: D
text: The movement of any animal away from potential trouble warns all of the other animals within the herd.
answer_key: B
explanation: 第2句话对应 A 选项，第3句话对应 C，第5和6句话对应 D，B 选项跟最后一句话中的”simply protecting itself” 冲突，故选 B。

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

tag: B
text: because

tag: C
text: unless

tag: D
text: therefore
answer_key: A
explanation: provided 在这儿是“如果“的意思，引导条件状语从句，选 A。

question_stem: According to paragraph 5, any member of a herd is most at risk from predators when
choices: tag: A
text: it is very young

tag: B
text: it becomes separated from the others

tag: C
text: the herd includes relatively few old or sick animals

tag: D
text: the herd begins to run
answer_key: B
explanation: 根据段落2-4句可知：捕食者会挑落单的猎物下手，因此选择 B。无论是体弱多病还是年幼，都是与群体 separate 的情况。

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 hunter or hunters are interested only in seizing their prey and not in any of the rest of the grazers in the herd.

tag: B
text: Because the hunters must concentrate on eating once they make a kill, the rest of the herd can then stop running and resume grazing.

tag: C
text: Once the hunters finish eating their prey, they lose interest in the rest of the herd that was left behind.

tag: D
text: Because they must also concentrate on eating, herd animals resume grazing as soon as the hunter or hunters seize their prey.
answer_key: B
explanation: 原句意思：一旦捕食者抓住猎物，它们就对其他食草动物失去了兴确，因为它们要专心吃，这时，食草动物们就不再跑了，那些落后的重新回到群里，大家一起继续吃草。A 选项说捕食者只对抓猎物感兴趣，对兽群中其他食草动物都不感兴趣，偷换了对原句子 interest 相关内容的概念；C 选项中的 finish eating their prey 跟原文不符合；D 选项的 they 指代 herd animals 是张冠李戴，因为专注于吃的是 hunters 不是 herd animals。