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Vitamin D helps bones and muscles grow strong and healthy. Low levels of vitamin D can lead to diseases such as rickets (佝偻病), mainly found in children. The thinning of bone is a common problem as people, especially women, get older.
But more and more research is suggesting that vitamin D might also help prevent many diseases. The easiest way to get vitamin D is from sunlight. The sun’s ultraviolet (紫外线的) rays react with skin cells to produce vitamin D. But many people worry about getting skin cancer and skin damage from the sun. As a result they stay out of the sun. Also, darker-skinned people produce less vitamin D than lighter-skinned people. The amount also decreases in older people and those living in northern areas that get less sunlight.
Not many foods naturally contain vitamin D. Foods high in this vitamin include oily fish and fish liver oils (鱼肝油). Farmed fish have only about one-fourth as much vitamin D as wild fish. Small amounts of vitamin D are found in beef liver, cheese and egg yolks. But most of the vitamin D in the American diet comes from foods such as milk.
Research in the last several years has shown that low levels of vitamin D may increase the risk of heart attacks in men and deaths from some cancers. More doctors are now having their patients tested for their vitamin D levels. But as research continues, some experts worry that if people take too much vitamin D, it might act as a poison. Also, skin doctors warn people to be careful with sun exposure because of the risk of skin cancer.
小题1:The first paragraph is mainly about 　　　　 .

A．the muscle diseases	B．the sources of vitamins
C．the importance of health	D．the functions of vitamin D

小题2:Which of the following people produce the least amount of vitamin D?______

A．Elderly darker-skinned people.	B．Young lighter-skinned people.
C．Elderly lighter-skinned people.	D．Young darker-skinned people.

小题3: People can get the most vitamin D from 　　　　 .

A．beef liver

B．cheese

C．wild fish

D．farmed fish

小题4:What can we infer from the text?_________
A. Being exposed to sunlight is the safest way to get vitamin D.
B. Lack of vitamin D can make people age quickly.
C. Proper vitamin D levels people need are still not determined now.
D. The vitamin D level is higher in men than in women.