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

fortitude

 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

选项中为什么要使用its numbers 而不是用its number

Thanks very much 

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fortitude

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fortitude

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fortitude

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fortitude

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fortitude

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fortitude

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fortitude

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fortitude

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