This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gastrointestinal System, Subject name: Physiology
Which of the following statements is true about protein digestion? 
Select and explain. 
A. Protein digestion stas with the salivary action, B. Trypsinogen is conveed to trypsin by the action of bicarbonate in duodenum, C. Trypsin and chymotrypsin are exopeptidases, D. 10%-20% protein digestion occurs by pepsin action in stomach.
D: 10%-20% protein digestion occurs by pepsin action in stomach.
Answer: D: 10%-20% protein digestion occurs by pepsin action in stomach. 
Explanation: - Saliva does not contain proteolytic enzyme; protein digestion begins in the stomach by the action of pepsin. - Trypsinogen is conveed to trypsin by the action of enterokinase in duodenum. - Trypsin and chymotrypsin are endopeptidases; carboxypeptidase and aminopeptidase are exopeptidases. - Of the total protein digestion, 10%-20% occurs by the action of pepsin.