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题库 / Prep2007E2-SC-21
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
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在CD之间选，实际上就是时态的区别
"learning how to synthesize the growth hormone, scientists had to remove it painstakingly" -- dont know if its wrong, but I would still prefer C.
because, of the word "had" which suggests two actions in the past..so ideally, it should be "learned".
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