请问OG251选项中为什么用its numbers而不用its number

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

Why can't I use" its number" instead of "its number" in selections A and B

Thank you very much

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