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
我选了D,看了解释还是不明白D为什么错？请指教。
解释说C中，it指代the growth hormone，所以正确。
D中it指代the growth hormone，不符合逻辑，所以不正确。
是在不明白D到底不符合逻辑在哪了？
多谢楼上的，我明白了。
a synthetic growth hormone不应该是从pituitary glands of human cadavers中painstakingly removed的。
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