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 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


Can anyone help me understand the difference between C and D

Thanks for your help