[求助]大全790,时态问题,不解,讨教!

The number of mountain gorillas is declining with such rapidity that the population is one-half in the twenty years between a count made by George Schaller in 1960 and the one made by Dian Fossey in 1980.

(A) with such rapidity that the population is one-half

(B) with such rapidity that the population was one-half

(C) so rapidly the population divided in half

(D) so rapidly that the population was halved（D）

(E) in such rapidity that the population is halved

so ..that...无问题,但是对于D项的时态有疑问:

The number of mountain gorillas **is** declining so rapidly that the population** was **halved in the twenty years between a count made by George Schaller in 1960 and the one made by Dian Fossey in 1980.

怎么理解?

如果时态优先,是否应该选E?

请教,谢先!

对.Between后的短语表示过去时间,动作在过去完成.应该这样理解.谢谢!

so xxx that xxx 要么表目的. 要么表结果.

在这里表目的显然是不对的.

那么如果是表结果的话.The number of mountain gorillas is declining (现在式) 其结果the population is one-half
            in the twenty years between a count made by George Schaller in 1960 and the one made by Dian Fossey in 1980 (过去式).

一个现在式怎么可能导致过去的结果呢? 虽然标准答案是D, 可是我觉得E更好. 虽然 in such rapdity 不如so that 好.

请NN解答...

2楼解释的很清楚了, 现在式表示一个ongoing, 但是数字的计算是在过去.

比如: AIDS一直在剥夺很多人的生命(ongoing), 在全球4500万的AIDS病人中2000万都已经死亡.(这里暗示着我必然是截至到我说话前的某一个时刻, 那么这个数据必然是更早的时间统计的, 自然用过去式)

thanks for your reply...though i'm still not convinced...