As food is processed throughout the digestive system, it is exposed to a variety of pH (acidic or base) levels. The stomach has a pH of 2 (a strong acidic content) due to the presence of hydrochloride acid (HCl), and the small intestine has a pH ranging from 7 to 9. HCl converts pepsinogen into pepsin, an enzyme that digests proteins in the stomach. Which of the following most likely happens to pepsin as it enters the small intestine?

A. It becomes inactive.

B. It begins to replicate.

C. It's shape changes to engulf large proteins.

D. It's activity increases to digest more proteins.