90. 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
(E) in such rapidity that the population is halved