Which of the following statements is true? A thin layer of chyme protects the lining of the digestive tract from being broken down by enzymes. Hydrochloric acid (HCl) begins the partial digestion of fats in the small intestine. Cystic fibrosis is an infectious disease that causes the muscles that control swallowing to malfunction. Heartburn often results when the gastroesophageal sphincter fails to close properly.