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.
这是一个错综时态复杂句。
主语The number of mountain gorillas谓语 is declining （山里面大猩猩的数量正在减少，这是描述现在的情况）so rapidly that the population was halved in the twenty years （减少的如此之快，怎么快？如此快以至于在过去的二十年里面数量减少拉一半。）between a count made by George Schaller in 1960 and the one made by Dian Fossey in 1980.（再说明是哪二十年？1960--1980。）
所以选B | 
