Before scientists learned how to make a synthetic growth hormone, removing it painstakingly in small amounts from the pituitary glands of human cadevers.

A)
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 sythetic growth hormone, scientists had to remove it painstakingly
E) learning how to synthesize the growth hormone, it had to be painstakingly removed by scientists