Before scientists learned how to make a synthetic growth hormone, removing it painstakingly in small amounts from the pituitary glands of human cadavers.
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Ascientists learned how to make a synthetic growth hormone, removing it painstakingly
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Bscientists had learned about making a synthetic growth hormone, they had to remove it painstakingly
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Cscientists learned how to synthesize the growth hormone, it had to be painstakingly removed
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Dlearning how to make a synthetic growth hormone, scientists had to remove it painstakingly
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Elearning how to synthesize the growth hormone, it had to be painstakingly removed by scientists
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正确答案: C