求教prep 语法一题> <关于指代的

zombiest

Before scientists learned how to make a synthetic growth hormone, removing it painstakingly in
small amounts from the pituitary glands of human cadaver

(A) scientists learned how to make a synthetic growth hormone, removing it painstakingly
(B) scientists had learned about making a synthetic growth hormone, they had to remove it
painstakingly
(C) scientists learned how to synthesize the growth hormone, it had to be painstakingly removed
(D) learning how to make a synthetic growth hormone, scientists had to remove it painstakingly
(E) learning how to synthesize the growth hormone, it had to be painstakingly removed by scientists  
答案是C 时间状语从句主句的主语和从句的主语可以不一致的嘛？为什么it在这里能指代the growth hormone?  谢~~~~~~

zombiest

谢啦~
D的话 因为it在这里变成指代a synthetic growth hormone 应该指代growth hormone~~

xiaoqiaoli

主语的主语是可以和从句的主语不一致的。。。

因为 scientists 是复数形式的，the growth hormone 是单数，It只能指代单数的啊。。。。

不过这道题为什么不选D呢？