求教 该用“its number” or“its numbers”？

crystal9006

OG251. 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    （A）
e)    extinction, now with numbers five times greater than
在我的印象中指名词的数量应该用 its number  ，a large number of  Ns,  为什么number有复数形式啊  还是我走火入魔啊  求教啊~~！

sheikh239

有非常详细的讨论 关于number和numbers 翻翻旧帖