【单选题】

Vitamins are organic compounds necessary in small amounts in the diet for the normal growth and maintenance of life of animals, including man.

They do not provide energy, (21) do they construct or build any part of the body. They are needed for (22) foods into energy and body maintenance. There are thirteen or more of them, and if (23) is missing a deficiency disease becomes (24)
Vitamins are similar because they are (25) of the same elements--usually carbon, hydrogen, oxygen, and (26) . nitrogen. They are different (27) their elements are arranged (28) , and each vitamin (29) one or more specific (30) in the body.
(31) enough vitamins is essential to life, (32) the body has no nutritional use for (33) vitamins. Many people, (34) , believe in being on the "safe side" and thus take extra vitamins. However, a (35) diet will usually meet all the body’s vitamin needs.
A．functions
B．mood
C．role
D．meaning