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
(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
尽管破解版里解释了为什么选C，可是我仍觉得D也行！为什么说D答案里it 指代 synthetic growth hormone不对呢？不然it指代的是什么呢？请大神指点~~~~~~~
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发表于 2011-11-29 16:03:23
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