求教

ericwilliam

21.(25520-!-item-!-188;#058&002357)

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
答案是C，可是D怎么排除？

清飞扬

C中的it指代the growth hormone，D中的it指代a synthetic growth hormone，所以从逻辑意思上来说，it应该指代the growth hormone，因为科学家需要learned to synthetic  growth hormone