How does the activation of pepsinogen in the stomach illustrate the role of zymogens in digestion?

Pepsinogen is activated by a decrease in temperature in the stomach.

Pepsinogen is activated by binding to carbohydrates in the stomach.

Pepsinogen is activated by the presence of lipids in the stomach.

Pepsinogen is activated to pepsin in the acidic environment of the stomach, aiding in protein digestion.