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.
such rapidity that the population is one-half
such rapidity that the population was one-half
--> "with " changes the meaning to absurdity by saying the act of declining includes "such rapidity"
(C) so rapidly the population divided in half
--> we need 'that' here to introduce the subordinate clause
(D) so rapidly that the population was halved
(E) in such rapidity that the population is
--> need past tense here because the act of getting half occurred in past.
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