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 错，没有主语
(B) scientists had learned about making a synthetic growth hormone, they had to remove it painstakingly 错about making a synthetic 太冗长了
(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 相对原文改动最小
(E) learning how to synthesize the growth hormone, it had to be painstakingly removed by scientists 错learning的逻辑主语是it ，不对
