【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 made of the same elements--usually carbon, hydrogen, oxygen, and 25 nitrogen. They are different 26 their elements are arranged differently, and each vitamin 27 one or more specific functions in the body.
28 enough vitamins is essential to life, although the body has no nutritional use for 29 vitamins. Many people, 30 , believe in being on the "safe side" and thus take extra vitamins. However, a well-balanced diet will usually meet all the body’s vitamin needs.
A．either
B．so
C．nor
D．never