Concept:Hydrochloric acid in the stomach creates an acidic environment needed for protein digestion and stops carbohydrate digestion begun in the mouth.Explanation:Ptyalin is salivary amylase that acts in the mouth and digests starch.Ptyalin requires a neutral or slightly alkaline medium to work.Hydrochloric acid makes the stomach contents acidic, which stops the action of ptyalin.It also activates pepsinogen to pepsin for protein digestion.Answer: C. stop the action of ptyalin