听力原文： New studies say that eating chocolate may be good for your heart. Research shows that eating chocolate may reduce the chance of developing heart disease by helping blood vessels remain clear and open. Chocolate is produced from beans that contain large amounts of natural substances called flavonoids. Flavonoids have been shown to protect the heart and the blood vessels from damage. Recent studies have linked foods that contain flavonoids with a reduced chance of developing heart disease. These foods include tea, red wine, apples, peanuts and chocolate. Research has shown that forty grams of milk chocolate contain about four-hundred milligrams of flavonoids. That is about the same amount as in a glass of red wine. The same amount of dark choc-olate contains more than eight-hundred milligrams of flavonoids. That is about the same amount as in a cup of black tea. A study by researchers at the University of Scranton in Pennsylvania found that the flavonoids in chocolate are more powerful than some vitamins in limiting the damage from fats in the blood. A German study recently showed that the flavonoids in chocolate also could stop damage to blood vessel walls. And a University of California study showed that flavonoids from chocolate produce nitric acid in the blood. Nitric acid helps protect the walls of blood vessels. A study by Cesar Fraga of the University of Buenos Aires in Argentina confirmed healthy activity in the blood after eating dark chocolate. Carl Keen of the University of California at Davis also carried out studies about flavonoids. The studies showed that foods high in flavonoids may help stop thickening of the blood that can lead to heart attacks or strokes. Mister Keen gave water, cocoa or non-alcoholic red wine to groups of ten men and women. Researchers tested their blood a few hours later. Both the wine and cocoa delayed the time it took for the blood to become solid. Only the cocoa protected platelets in the blood from becoming too sticky and breaking. What is the main idea of the passage?