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
请问D错在哪里啊,答案是C. |