[求助]一道语法题

The number of mountain gorillas is declining with such rapidity that the population is one-half in the twenty years between a count wade 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 opulation was one-half

(C) so rapidly the population divided in half

(D) so rapidly that the population was halved

(E) in such rapidity that the population is halved

答案是（D), 不太明白，为什么前面是现在进行时，后面是一般过去时？

 in the twenty years between a count wade by George Schaller in 1960 and the one made by Dian Fossey in 1980

这一段应该能说明问题了吧，前面是表示一般状态，后面相当于一个时间段的案例

请LZ在发贴前阅读发贴规则，谢谢配合啦。

    

不好意思，答案应该是D。