请教一道语法题

HermanLu

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 have to be painstakingly removed by scientists

richtung

这个简直就是不考语法考句意的典型代表啊！也错选了D。

http://www.manhattangmat.com/forums/before-scientists-learned-how-to-make-a-synthetic-growth-t637.html