Explanation

Pepsin works in the acidic medium of the stomach (pH around 2), while trypsin acts in the alkaline small intestine (pH around 8). Salivary amylase works near neutral pH and is inactivated by stomach acid. Thus trypsin and amylase are the pair that do not work at acidic pH. Chymotrypsin, like trypsin, is also an alkaline-medium enzyme.