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I also did not pay attention and fell for D. But Walker pointed it out correctly and it makes all the sense now.
"Learning how to synthesize the growth hormone" is better than "Learning how to make a synthetic growth hormone"
Also, 'it' in option C correctly refers to 'growth hormone'
D incorrectly says that "scientists had to remove it [synthetic growth hormone] painstakingly"....which is not correct.
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. a scientist learned how to synthesize the growth hormone, it had to be painstakingly removed - CORRECT 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