OG16 Verbal SC 20关于fivefold用法的问题

阿霖

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

正确答案是A，我想请问一下C选项，OG对于C的解释是“Fivefold what they were” is awkward and nonstandard and implies that there are now more numbers, rather than more gyrfalcon
我不是很理解，希望大神能指点一下。
另外请问fiveofld正确的用法应该是什么呢？

RON说即使把C中的their改成its也是不对的，我也没有理解他的说法，我同时把他的解释贴在下面，希望大神能帮忙解释一下。
this choice is grammatically ok, but it doesn't make sense -- you've created a modifier that doesn't actually modify anything.

the point of "its numbers are now fivefold..." is to create a contrast with the previous clause (which says that the bird almost went extinct at one point), not to describe/modify it. so you can't use a modifier here.

note that the correct answer does not use one, either.