求教：og10th 251题 为什么表数量要用numbers，然后是复数？

winnielxx

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. .
extinction; its numbers are now five times greater than
extinction; its numbers are now five times more than
extinction, their numbers now fivefold what they were
extinction, now with fivefold the numbers they had
extinction, now with numbers five times greater than


实在是不懂啊，为什么答案a表示数量5倍多要用numbers are？实在不明白，请大家帮帮忙啊