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.

It seems to me that both C and D are correct. Can someone explain why one is more correct than the other?