Why Was the Glass-Steagall Act a Key Piece of Legislation

Why was the Glass-Steagall Act a key piece of legislation?

A) It took on the debt of commercial banks to ensure their solvency and financial health.
B) It established a gold standard to shore up the strength of the American dollar.
C) It banned commercial banks from involvement in buying and selling stocks, and set up the FDIC.
D) It proved to be a temporary financial measure that did not survive beyond the Great Depression.