Proteins Are Able to Act as Buffers Because Their -COOH

Proteins are able to act as buffers because their -COOH groups can _____________, and their -NH₂ groups can ______________.

A) bind or release H⁺ when pH is decreased or elevated; bind or release H⁺ when pH is decreased or elevated
B) release H⁺ when pH is elevated; bind H⁺ when pH is decreased
C) bind H⁺ when pH is decreased; release H⁺ when pH is elevated
D) release H⁺ when pH is decreased; bind H⁺ when pH is elevated