Carnivorous mammals can endure what would otherwise be lethal levels of body heat because they have a heat-exchange network which kept the brain from getting too hot.
(A) which kept
(B) that keeps
(C) which has kept
(D) that has been keeping
(E) having kept
OG解释：
Verb form; Rhetorical construction
The use of the past tense (kept) is incorrect because a current situation is discussed; the present tense (keeps) is consistent with the other verbs in the sentence. In (A) and (C), which introduces a restrictive clause. Some writers follow the convention that which can only be used
for nonrestrictive clauses, but insistence on this rule is controversial, and both (A) and (C) can be rejected on other grounds.
A Kept is the wrong tense.
B Correct. The verb keeps indicates a current situation and is consistent with the other verbs in the sentence. The sentence is clear
and concise.
C Mistaken shift in tense: In this sentence the present tense expresses a timeless general principle; in contrast, has kept indicates a more definite context and time period and suggests that the heat-exchange network may no longer have this effect.
D Has been keeping is the wrong tense.
E Having is awkward and imprecise; kept is the wrong tense.
我的疑问是c的解释中，说has kept作为现在完成时表达了definite的时间段，而且影响不会持续。但是Gmat前面对现在完成时的解释又是:decribe an action that occured at an indefinite time in that past. 国外网站上Ron对现在完成时的解释也是the present perfect gives the impression that something has happened in the past, in a way that
* has persisted up to the present,
* still affects the present even though it's over, OR
* is over, but could happen again.
求救啊求救，为何此题C中这样解释啊，寝食难安~~~~万分谢谢！另外，D中现在完成进行时其实也可以说得通吧？ 作者: vertex顶点 时间: 2013-9-5 11:39
OG没有说到的一点，即which做先行词修饰定语从句只可能在非限制性定语从句中，即前面要有逗号才行。 这才是为什么这道题在第12题的原因，这属于入门级题目，不要想太多。 至于现在完成时，我给你一个整体的解释：
现在完成时仅仅出现在以下两种情况：
1. 表示“已经”（I have eaten lunch，我已经吃过饭了，意思就是不用再请我吃了，吃不下了）
*在没有since这样的现在完成时标志词的时候用现在完成时，必须表达的是“已经”的意思，更深层的含义是已经完成了不会再继续了。（所以evidence has suggested就不对，因为它代表着evidence不会再suggest了的含义）
2. 表示“到现在为止”，指到现在为止的情况下最好有加时间，从什么什么时候到现在为止，例子：Few people have fully recovered from this horrible disease since 1945（从1945年到现在为止，只有很少一部分人从这个疾病中痊愈了） 作者: 刘下巴 时间: 2013-9-6 19:00
谢谢解答，Og里面提到了which这点，不过还是有争议的。我的困扰就是对现在完成时的解释太多了，我不知道是哪种了。。。
！另外，D中现在完成进行时其实也可以说得通吧？