A．They are studying why people expend calories.B．They are studying how long people should spend on exercising.C．They are studying what is the most important way of keeping fit.D．They are studying why some people get fat and others don’t.
A．They are studying why people expend calories.
B．They are studying how long people should spend on exercising.
C．They are studying what is the most important way of keeping fit.
D．They are studying why some people get fat and others don’t.