Sentence Correction

Before learning how to make a synthetic growth hormone, scientists had to remove it painstakingly in small amounts from the pituitary glands of human cadavers.
A. learning how to make a synthetic growth hormone, scientists had to remove it painstakingly
B. scientists learned how to make a synthetic growth hormone, they removed it painstakingly
C. scientists learned how to make a synthetic growth hormone, they had to remove it painstakingly
D. scientists learned how to synthesize the growth hormone, it had to be painstakingly removed
E. learning how to synthesize the growth hormone, it had to be painstakingly removed by scientists

Toponepercent GMAT wrote:Before learning how to make a synthetic growth hormone, scientists had to remove it painstakingly in small amounts from the pituitary glands of human cadavers.
A. learning how to make a synthetic growth hormone, scientists had to remove it painstakingly
B. scientists learned how to make a synthetic growth hormone, they removed it painstakingly
C. scientists learned how to make a synthetic growth hormone, they had to remove it painstakingly
D. scientists learned how to synthesize the growth hormone, it had to be painstakingly removed
E. learning how to synthesize the growth hormone, it had to be painstakingly removed by scientists

Synthetic means artificial.
A. Before learning how to make a synthetic growth hormone, scientists had to remove it painstakingly...from...human cadavers.
B. Before scientists learned how to make a synthetic growth hormone, they removed it painstakingly...from...human cadavers.
D. Before scientists learned how to synthesize the growth hormone, it had to be painstakingly removed...from...human cadavers.
In these options, it seems to refer to a synthetic growth hormone, implying that an ARTIFICIAL hormone was removed from HUMAN cadavers.
This meaning is nonsensical.
Eliminate A, B and D.

E: Before learning how to synthesize the growth hormone, it had to be painstakingly removed.
Here, learning seems to refer to it -- a pronoun standing in for the growth hormone -- conveying that THE GROWTH HORMONE was LEARNING.
This meaning is nonsensical.
Eliminate E.

The correct answer is C.