A、Vitamin A. B、Vitamin B. C、Vitamin C. D、Vitamin D. D信息明示题。短文中提到Vitamin D is the only vitamin which the body can make for itself

About fifty different vitamins have been identified, and a deficiency in many of these can lead to illness. Vitamin A is most important for good eyesight. Liver contains a considerable amount of vitamin A, but vitamin A is also found in fish, meat, milk, and some fruits and vegetables.
    Vitamin B in fact consists of twelve different chemicals, which are found in egg, cheese and vegetables. If a person has an inadequate amount of vitamin B in his or her diet, this may particularly affect the skin, the nervous system and the heart.
    Vitamin C helps to heal injuries. Some doctors believe that large quantities of vitamin C helps people avoid colds. Fruits and uncooked vegetables are rich in vitamin C, but when they are overcooked, or left for a long time, they lose most of their vitamins.
    Vitamin D is essential for the growth of bones and is found in fish, liver and milk. Vitamin D is the only vitamin which the body can make for itself, but it can only do this if there is sufficient sunlight. A lack of both sunlight and vitamin D can result in disease called rickets, which causes bones to soften.
    A well-balanced diet will provide an adequate amount of vitamins.
Questions 19 to 21 are based on the passage you have just heard.
19. Which of the following statements is true?
20. Which vitamin can be made by the human body?
21. Which of the following statement is implied by the speaker?