OG10-251 中的 its numbers

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
        

为什么用：its numbers 而不是 its number