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.

The correct answer is: Heartburn often results when the gastroesophageal sphincter fails to close properly.

The gastroesophageal sphincter is a muscle located at the bottom of the esophagus that has to be relaxed in order to allow food to flow down into the stomach. After it contracts (tightens), the food and the stomach acid can't flow back up into the esophagus.

When the gastroesophageal sphincter fails to close properly acid reflux occurs and causes the heartburn.