标题: 求教SC [打印本页] 作者: witness236 时间: 2012-4-22 17:24 标题: 求教SC 21. (25520-!-item-!-188;#058&002357) 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 这道题给出的答案是C，我想知道D选项有什么问题