Before researchers learned how to make a synthetic metabolic enzyme, extracting it meticulously in tiny amounts from the pancreases of human cadavers.
(A) researchers learned how to make a synthetic metabolic enzyme, extracting it meticulously
(B) researchers had learned about making a synthetic metabolic enzyme, they had to remove it meticulously
(C) researchers learned how to synthesize the metabolic enzyme, it had to be meticulously extracted
(D) learning how to make a synthetic metabolic enzyme, researchers had to extract it meticulously
(E) learning how to synthesize the metabolic enzyme, it had to be meticulously extracted by researchers

I am not able to understand why OA is C.