sec13-23,时态问题

23.   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

这题都没人问过.看来我够弱智的..哎..  

问个时态, 这题时态前面is 后面 was.. 怎么理解呀?   is declining so...that....was...? 感觉意思怪