11. 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. scientist learned how to synthesize合成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 have to be painstakingly removed by scientists 给的答案是C，我认为是D。因为我觉得主动优于被动。大家看呢？ | 
