790. 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

Solution:

Idioms

“with such rapidity” and “in such rapidity” are unidiomatic and hence, A, B, and E are incorrect.
C wrongly suggests that the “population divided in half”. Furthermore, “that” is missing before “population”.
In D, “so rapidly…” is idiomatic through “...so + adjective/adverb + that..” structure. “was halved” implies that the population has already been halved by outside forces such as poachers, diseases, etc. Hence, D is the best choice.

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