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 请问这题为什么选C 不选D呀? 谢谢 |