请教一道语法题！！！

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

请问这题为什么选C  不选D呀？   谢谢

为什么D中的it只能指"a synthetic growth hormone", a synthetic是修饰 growth hormone的呀，所以应该也指代growth hormone呀。 为什么呀？

主要是it指代的问题 ， 我已经mark出来了，楼主再看看，体会一下

两个it的指代是不同的，导致意思变化， 而D是逻辑不通的