单选题

　　[完成句子]Certainly there are millions who need to lose weight. But there are also millions who only imagine they need to. Compulsive and continuous dieting， not to mention eating disorders， shows that some of us will do anything to reduce our bodies down to the currently desirable shape. But is being underweight really desirable？

　　A great many people go on a diet to__________.

　　A、keep fit

　　B、have a body shape that all people will admire

　　C、prolong life span

　　D、get ride of digestive system diseases