A、200. B、400. C、600 D、800 C短文在最后提到，到70岁时每天需要400国际单位(IU)的维生素D，71岁及以上的人就需要600IU。题目问80岁的老人，故选C。

Vitamin D helps bones and muscles grow strong and healthy.
    The easiest way to get vitamin D is from sunlight. The ultraviolet rays react with skin cells to produce the vitamin. But many people worry about skin cancer and skin damage from the sun.
    Also, darker skinned people produce less vitamin D than lighter skinned people. Production also decreases in older people and those living in northern areas that get less sunlight.
    Not many foods naturally contain vitamin D. Foods with high levels include oily fish such as salmon, tuna and mackerel, and fish liver oils. Boston University researchers reported last year that farmed salmon had a lot less vitamin D than wild salmon.
    Small amounts of D are found in beef liver, cheese and egg. And some people take dietary supplements containing the vitamin. But most of the D in the American diet comes from foods like milk with the vitamin added.
    These days, more doctors are testing for vitamin D levels in their patients. But as research continues, some experts worry that if people take too much D, it might act as a poison. Also, skin doctors warn people to be careful with sun exposure.
    How much vitamin D does a healthy person need? Nutrition experts who advise the American government set the current recommendations in 1997. The daily amount is two hundred international units from birth through age 50. Then it rises to four hundred international units through age seventy, and six hundred for those seventy-one and older. But some groups say these amounts are not high enough.
16. According to the speaker, what is the easiest way to get Vitamin D?
17. According to the speaker, who produces the most Vitamin D?
18. What do some experts worry about?
19. How much Vitamin D does an 80-year-old person need every day?