72. His studies of ice-polished rocks in his Alpine homeland, far outside the range of present-day glaciers, led Louis Agassiz in 1837 to propose the concept of an agein which great ice sheets had existed in now currently temperate areas.
(A) in which great ice sheets had existed in now currently temperate areas
(B) in which great ice sheets existed in what are now temperate areas
(C) when great ice sheets existed where there were areas now temperate
(D) when great ice sheets had existed in current temperate areas
(E) when great ice sheets existed in areas now that are temperate
这个题，我真的不知道为什么选B啊。。。时态让我好纠结。。
- 77. A 1972 agreement between Canada and the United Statesreduced the amount of phosphates（n. 光气，磷酸盐（phosphate的复数形式）） that municipalities（n. 市民，自治市；市政公债（municipality复数形式））had been allowed to dumpinto the Great Lakes.
(A) reduced the amount of phosphates that municipalities had been allowed to dump
(B) reduced the phosphate amount that municipalities had been dumping
(C) reduces the phosphate amount municipalities have been allowed to dump
(D) reduced the amount of phosphates that municipalities are allowed to dump
(E) reduces the amount of phosphates allowed for dumping by municipalities
这个题，我能选对，但我问自己为什么，我说不出。就是感觉。。我还是想知道每题怎么对怎么错的啊。。
这个题，我用排除法排除了B，C，E。就在A,D中徘徊。。我还是不明白时态啊。。求大神，这个gmat的解释我没看懂！
- 77. A 1972 agreement between Canada and the United Statesreduced the amount of phosphates（n. 光气，磷酸盐（phosphate的复数形式）） that municipalities（n. 市民，自治市；市政公债（municipality复数形式））had been allowed to dumpinto the Great Lakes.
(A) reduced the amount of phosphates that municipalities had been allowed to dump
(B) reduced the phosphate amount that municipalities had been dumping
(C) reduces the phosphate amount municipalities have been allowed to dump
(D) reduced the amount of phosphates that municipalities are allowed to dump
(E) reduces the amount of phosphates allowed for dumping by municipalities
这个题，我能选对，但我问自己为什么，我说不出。就是感觉。。我还是想知道每题怎么对怎么错的啊。。
这个题，我用排除法排除了B，C，E。就在A,D中徘徊。。我还是不明白时态啊。。求大神，这个gmat的解释我没看懂！
急寻大N给予时态答法的心得！！！！跪求，抱大腿啊！！！！！