题目:
Before scientists learned how to make a synthetic growth hormone, removing it painstakingly in small amounts from the pituitary glands of human cadavers.
选项:
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
答案:
C