【正确答案】Vital food falls into three categories. The first one is Carbohydrate. It exists widely in animals and plants and consists of sugar, starch and cellulose. Carbohydrate provides energy for people and also synthesizes glucose and enzymes necessary in biological process. The second one is fat. As a more "concentrated" energy than carbohydrate, fat functions as stored energy instead of that which is instantly usable. The third one is protein. Composed of macromolecules containing carbon, hydrogen , oxygen, or nitrogen, protein is mainly employed to form structure or material necessary for life rather than to provide energy.