单选题 .  Cocoa contains chemicals called flavanols (黄烷醇) that may help reduce the risk of strokes and heart attacks by lowering blood pressure and bad cholesterol levels and improving the body's use of insulin (胰岛素).
    Washington chocolate producer Steve Kumar said that is one reason dark chocolate is increasingly popular among his customers. "A lot of people are attracted by dark chocolate because they just decide they love it, and they also have been reading all about the health benefits of dark chocolate," said Kumar.
    But those benefits have not been confirmed by studies involving large numbers of people. Also, during the production of chocolate, many flavanoids are destroyed, while sugar and fats are added to contribute to flavor.
    Scientists now want to learn the benefits of flavanols in their pure form. They plan a four-year study of 18,000 adults, who will take pills of pure cocoa flavanols, in what is being called the largest test of its kind. Jo-Ann Manson, of the Brigham and Women's Hospital in Boston, is the lead researcher. "This pill of cocoa flavanols will avoid having the calories and the sugar and the saturated fat found in chocolate," said Manson. The pills also won't have any taste. Participants, divided in two groups, will take two identical pills a day. One group's pills will contain flavanols, while the others will get placebo (安慰剂) pills. "The amount of chocolate that it would take in order to have this amount of cocoa flavanols would be more than ten times the amount that people would ordinarily eat," said Manson.
    Whatever the outcome of the study, Kumar said he's not worried about his business. "People like the experience of actually biting into a piece of chocolate, whatever their favorites are," he said.
    The flavanol benefits test is still in the early stage, so scientists are not sure when the participants will be handed their first chocolate pills.16.  What chemical in cocoa may do good to the heart according to the passage? ______