Before scientists learned how to make a synthetic growth hormone, removing it painstakingly in small amounts from the pituitary glands of human cadavers.
scientists learned how to make a synthetic growth hormone, removing it painstakingly
scientists had learned about making a synthetic growth hormone, they had to remove it painstakingly
scientists learned how to synthesize the growth hormone, it had to be painstakingly removed
learning how to make a synthetic growth hormone, scientists had to remove it painstakingly
learning how to synthesize the growth hormone, it had to be painstakingly removed by scientists
CD 区分:
it 做代词,则需要代指全面名词包括其定语,那么在C中,it 代指growth hormone; D中代指synthetic growth hormone(合成的)。所以D错
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