Ruth's Personal Trainer Tells Her Not to Walk So Fast

Ruth's personal trainer tells her not to walk so fast on the treadmill that walking becomes uncomfortable.Discomfort is her body's signal that she is not getting enough oxygen to burn fat.Instead,it begins to burn the simple sugars that are immediately available in her blood,called

A) glucose.
B) cholecystokinin (CCK) .
C) leptin.
D) insulin.