Pepsin, a common enzyme found in the stomach, is most active at ph values between 0.5 and 2.5. it works slowly between ph values of 2.6 and 3.4. beyond a ph of about 3.5, it becomes completely inactive. why does this happen? a) the enzyme inactivates because it melts into a liquid when it is not at the right ph. b) the protein chain in the enzyme is denatured, permanently inactivating it as the ph approaches 7. c) the enzymes slow down, because changing the ph changes the temperature as well, and affects the reaction. d) the enzyme's active site contracts into the wrong shape, but this is always reversible if the right ph is restored.