Play is easier todefinewith examples than with concepts. In any case, in animals it consists of leaping, running, climbing, throwing, wrestling, and other movements, eitheralong, with objects, or with other animals. Depending on thespecies, play may beprimarilyfor socialinteraction,exercise, orexploration. One of the problems in providing acleardefinitionof play is that it involves the same behaviors that take place in other circumstance--dominance,predation, competition, and real fighting. Thus, whether play occurs or not depends on the intention of the animals, and the intentions are not alwaysclearfrom behaviors alone.

Play appears to be a developmentalcharacteristicof animals withfairlysophisticatednervous systems, mainly birds and mammals. Play has been studied most extensively in primates and canids (dogs). Exactly why animals play is still a matter debated in the researchliterature, and the reasons may not be the same for everyspeciesthat plays. Determining the functions of play is difficult because the functions may be long-term, withbeneficialeffects not showing up until the animal's adulthood.

Play is not withoutconsiderablecosts to theindividualanimal. Play is usually very active, involving movement inspaceand, at times, noisemaking. Therefore, it results in the loss offuelor energy that might better be used for growth or for building up fat stores in a young animal. Anotherpotentialcostof thisactivityis greaterexposureto predators since play is attention-gettingbehavior. Great activities also increase the risk of injury in slipping or falling.

The benefits of play mustoutweighcosts, or play would not have evolved, according to Darwin' stheory. Some of thepotentialbenefitsrelatedirectly to the healthydevelopmentof the brain and nervous system. In one research study, two groups of young rats were raised under different conditions. One group developed in an "enriched" environment, which allowed the rats tointeractwith other rats, play with toys, and receivemazetraining. The other group lived in an "impoverished" environment inindividualcages in a dimly lit room with littlestimulation. At the end of the experiments, the results showed that the actual weight of the brains of the impoverished rats was less than that of those raised in the enriched environment (though they were fed the same diets). Other studies have shown that greaterstimulationnot only affects the size of the brain but also increase the number of connections between the nerve cells. Thus, active play may provide necessarystimulationto the growth of synaptic connections in the brain, especially the cerebellum, which isresponsibleformotorfunctioning and movements.

Play also stimulates thedevelopmentof themuscletissues themselves and may provide the opportunities to practice those movements needed forsurvival. Preyspecies, like young deer or goats, for example, typically play by performing sudden flight movements and turns, whereaspredatorspecies, such as cats, practice stalking, pouncing, and biting.

Play allows a young animal toexploreits environment and practice skill incomparativesafety since the surrounding adults generally do notexpectthe young todealwith threats or predators. Play can also provide practice in social behaviors needed forcourtshipand mating. Learningappropriatesocial behaviors is especially important forspeciesthat live in groups, like young monkeys that needed to learn to control selfishness andaggressionand to understand the give-and-take involved in social groups. They need to learn how to bedominantandsubmissivebecause each monkey might have to play either role in the future. Most of these things are learned in the long developmental periods that primates have, during which theyengagein countless play experiences with their peers.

There is a danger, of course, that play may be misinterpreted or not recognized as play by others, potentiallyleadingtoaggression. This is especially true when play consists of practicing normalaggressiveorpredatorbehaviors. Thus, manyspecieshave evolvedclearsignals todelineateplayfulness. Dogs, for example, will wag their tails, get down their front legs, andsticktheir behinds in theairtoindicate"what follows is just for play."

Questions:
question_stem: According to paragraph 1, why is play difficult to define?
choices: tag: A
text: Play must be defined with concepts, not examples.

tag: B
text: Play behavior often looks like nonplay behavior

tag: C
text: Play often occurs in the presence of animals that are not playing

tag: D
text: Play occurs independently of an animal’s intentions
answer_key: B
explanation: 。根据困难定位到段落第四句“One of the problems in providing a clear definition of play is that it involves the same behaviors that take place in other circumstance --dominance, predation, competition, and real fighting.”意思是：为玩耍提供一个清晰的定义非常困难，因为玩耍会包括一些在其他情况下发生的行为，比如统治、捕食、竞争和战斗。所以答案选 B。

question_stem: According to paragraph 2, which of the following presents a particular challenge to researchers who study play behavior in animals
choices: tag: A
text: The delay between activities and the benefits the animal derives from them.

tag: B
text: The difficulty in determining which animal species play and which do not.

tag: C
text: The fact that for most animals, there is no clear transition from youth to full adulthood.

tag: D
text: The lack of research on the play behavior of animals other than canids and primates.
answer_key: A
explanation: 正确答案 A。 看第二段的最后两句。倒数第二句说动物玩耍的原因仍然在被 debating，后面说确定玩耍的作用（function）非常难，那也就对应了原题的 challenge。Because 后面出现了原因，说这些 function 是长的，其 beneficial 要到动物成年才出现。所以跟 A 选项吻合。B、C 选项原文没提到。D 选项也和问题无关。

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

tag: B
text: Practical

tag: C
text: Eventually

tag: D
text: Significant
answer_key: D
explanation: 正确答案：D。Considerable：重要的，值得考虑的。选项 A 是最初的，B 是实用的，C 是最终的，D 是重大的，有意义的。所以 D 符合，另外后文提到了各种 cost 和 loss。所以可以理解 considerable cost 可以理解为一定量的损失或者风险。

question_stem: According to paragraph 3, each of the following is a cost to animals that engage in play EXCEPT
choices: tag: A
text: exposure to predators

tag: B
text: a buildup of fat stores

tag: C
text: a loss of fuel that could be used for growth

tag: D
text: risk of injury from slipping or falling
answer_key: B
explanation: B  从 Therefore 开始后面的都是 potentialcost。三种 cost 已经用红色字体标出。B 选项看似提到了，但是原文是说失去了 fat store 的机会，而不是因为 play 而 build up 了。所以 B 错。

question_stem: Why does the author include the comment “though they were fed the same diets”?
choices: tag: A
text: To show why rats living in impoverished environments need less food than those living in enriched environments

tag: B
text: To eliminate the possibility that differences in diet were responsibly for observed differences in brain weight

tag: C
text: To emphasize the point that rats were fed only the amount of food needed to keep them alive

tag: D
text: To suggest that rats fed the same diet have smaller brains than those fed a varied food
answer_key: B
explanation: B。 这里提到的试验的结果。前面从 in one research of study 开始描述这个试验，提到了两个对比组，under different conditions。后面是在说结果，括号里面的内容是对结论的解释说明。A,C,D 三个选项一点儿都不沾边。只要高中学过一点数理化就知道这个是控制变量的试验，所以答案选 B

question_stem: Paragraph 4 supports which of the following statements about an animal’s brain.
choices: tag: A
text: The heavier the brain, the richer the environment in which the animal was raised.

tag: B
text: The younger the animal, the harder it is to develop new connections between nerve cells.

tag: C
text: The larger the animal, the harder it is to develop new connections between nerve cells.

tag: D
text: The larger the animal’s cerebellum, the larger will be the animal’s nerve cells.
answer_key: A
explanation: 正确答案 A。 原文中从 other studies 开始有描述了另外的观点。说 greater stimulation 不仅会影响大脑的大小，也会增加神经细胞间的连接的数量，而这个 stimulation 是由 active play 提供的。根据上文，环境越复杂，active play 就越多，因此 stimulation 就越多，导致动物的大脑更大，也就更重。所以 A 选项符合这个观点。BCD 都没提到。

question_stem: According to paragraph 5, why might play behavior of prey species be different from those of predator species?
choices: tag: A
text: Unlike predator species, prey species use play to prevent inappropriate social behaviors, such as biting.

tag: B
text: Some prey species are physically incapable of certain types of predator movements.

tag: C
text: The survival of each species type is linked to particular sets of muscular movements.

tag: D
text: Predator species have more opportunities to practice play behaviors than prey species.
answer_key: C
explanation: 正确答案 C。这道题的关键词在于 needed for survival。因为 prey species 和 predator species 对于 survival 的要求不同，所以导致他们 play behavior 也不同。所以选 C。A，B，D 原文都没提到。

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

tag: B
text: temporary

tag: C
text: sufficient

tag: D
text: complete
answer_key: A
explanation: 正确答案 A。comparative 是相对的，相当的。A 是相对的；B 是暂时的；C 是充足的；D 是完成的。所以选择 A。这里 C 可能会是迷惑选项，因为其意思带入原文也通顺，但是失去了原词的“比较”的那层意思。

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: Only monkeys that have learned to control their selfish and aggressive behaviors can be involved in social groups.

tag: B
text: Selfish and aggressive animals like monkeys live in groups in order to practice appropriate social behaviors.

tag: C
text: Monkeys and other social animals need to learn behaviors appropriate for their social groups.

tag: D
text: Some monkeys are naturally too selfish and aggressive to understand the give-and-take of social groups, so they learn such important behaviors while young.
answer_key: C
explanation: 正确答案 C。 这道题中的原句主干是 learning social behavior is important，特别是对于群居的动物。然后用了 monkey 做例证。A 选项只提到了 monkeys 要控制自己才能融入群体，但没提到主干部分。B 选项说自私和攻击性强的动物药学 social behavior，这就把其他动物排除在外了，改变了原句的意思。C 正确，不仅提到了 monke y 也说到了其他的 social animals（which means species that live in groups）为了融入群体要学 social behavior。 D 是错误选项，原文并没说猴子太自私以至于不能理解 give and take…与原文矛盾，不选。

question_stem: What can be inferred from paragraph 6 about the role of adults in play activities of the young?
choices: tag: A
text: Adults help their young learn to become dominant within the social group.

tag: B
text: Young animals learn how to play from the adults within their social group.

tag: C
text: Adults allow the young to engage in play behaviors within a protected, safe environment.

tag: D
text: The long developmental period of some animals allows adults more time to teach their young how to deal with the threats of predators.
answer_key: C
explanation: 正确答案：C。 根据 young & adult 定位到第六段第一句。说成年动物要让年轻的动物在相对安全的环境下玩儿。 因此可以得到答案 C 正确。A 原文没有说让 young 去学怎么 dominant，B 原文虽然提到 social group 但没提到是从 adults 身上学习。D 原文完全没有相关描写。

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

tag: B
text: possibly

tag: C
text: unfortunately

tag: D
text: quickly
answer_key: B
explanation: 正确答案：B。potentially 潜在地。A 市毫无疑问地；B 是可能地；C 是不幸地；D 是快速地。前半句中有 May be，表达了一中可能性，后半句没有转折，应该是接上上半句往下说的。所以意思应该想近。

question_stem: According to paragraph 7, how do some animals ensure that other animals understand that they are just playing?
choices: tag: A
text: By playing only with animals who are not predator

tag: B
text: By avoiding any aspects of the play behavior that are dangerous

tag: C
text: By practicing nonaggressive and non-predatory behaviors

tag: D
text: By using a set of signals that occurs only in play
answer_key: D
explanation: 这一段前半部分一直在描述说有动物之间会误以为是真的 aggression，中间出现了 thus 证明后面会说解决的方法。这里说解决方法是用 clear signals 去表达 playfulness。所以答案是 D