Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 30 to 34.

Vitamins, taken in tiny doses, are a major group of organic compounds that regulate the mechanisms by which the body converts food into energy. They should not be confused with minerals, which are inorganic in their makeup. Although in general the naming of vitamins followed the alphabetical order of their identification, the nomenclature of individual substances may appear to be somewhat random and disorganized.. Among the 13 vitamins known today, five are produced in the body. Because the body produces sufficient quantities of some but not all vitamins, they must be supplemented in the daily diet. Although each vitamin has its specific designation and cannot be replaced by another compound, a lack of one vitamin can interfere with the processing of another. When a lack of even one vitamin in a diet is continual, a vitamin deficiency may result.

The best way for an individual to ensure a necessary supply of vitamins is to maintain a balanced diet that includes a variety of foods and provides adequate quantities of all the compounds. Some people take vitamin supplements, predominantly in the form of tablets. The vitamins in such supplements are equivalent to those in food, but an adult who maintains a balanced diet does not need a daily supplement. The ingestion of supplements is recommended only to correct an existing deficiency due to unbalanced diet, to provide vitamins known to be lacking in a restricted diet, or to act as a therapeutic measure in medical treatment. Specifically, caution must be exercised with fat-soluble substances, such as vitamins A and D, because, taken in gigantic doses, they may present a serious health hazard over a period of time.

According to the passage, vitamins are _______.