[求助]OG 251，不太理解og的解释，帮忙！

251. The gyrfalcon, an Arctic bird of prey, has survived a close brush with extinction; its numbers are now five times greater than when the use of DDT was sharply restricted in the early 1970's. .

(A)  extinction; its numbers are now five times greater than

(B)  extinction; its numbers are now five times more than

(C)  extinction, their numbers now fivefold what they were

(D)  extinction, now with fivefold the numbers
                    they had

(E)   extinction, now with numbers five times
                    greater than

A, the best choice, uses a singular pronoun, its, to refer to the singular antecedent The gyrfalcon, and it properly uses the construction its numbers are now ... greater than. In B, the construction
                        its numbers are ... more is not idiomatic: there are more birds, but not more numbers. Choices C and D use a plural pronoun, their or they, to refer to a grammatically singular antecedent, The gyrfalcon. Choices D and E wrongly use a phrase introduced by now with to modify The gyrfalcon. In both choices, the phrase confusingly seems to parallel with extinction; a new clause with a present tense verb is needed to state what the gyrfalcon's numbers are now.