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

fortitude

up

fortitude

up

诸神的黄昏

以下是引用fortitude在2007-9-28 0:26:00的发言：

 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 

number 数量;总数[C][(+of)]
注意，是可数名词。

danielsu

fortitude 发表于 2007-9-28 00:26
 The gyrfalcon, an Arctic bird of prey, has survived a close brush with extinction; its numbers ...

看一下！