3. D。细节题 从 …explorers have reported being free from colds until coming into contact again with infected people from the outside  中可以知道,这些探险者是因为与外界接触而感冒的。

 

 

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  Zinc(锌) is one of about 20 elements necessary for good health. It is present in the body in a very small amount but makes it possible for important chemical actions to be carried out in cells. Scientists are just beginning to discover how zinc and other elements work.

  Researchers first studied zinc as a way to speed recovery. Dr. Waltor Powys formerly with the United States Air Force noted that the laboratory animals recovered faster when some matters were added to their food. One of these matters was zinc. Dr. Powys later tested the zinc treatment on an American airman recovering from medical operations. The airman who received zinc sulphate(硫酸盐) recovered in half of the usual time. Other doctors studied persons who had a number of strange disorders. The doctors learned that all of these problems were caused by lack of zinc in the body. They treated these disorders successfully by giving the patients zinc sulphate.

  The amount of zinc in the body can be found by examining blood and hair. A lack of this important element is not uncommon among people in both industrial and developing countries. Zinc is found in most high protein(蛋白质) foods such as meat, milk, fish and eggs. It is also found in whole grains. But many people do not eat enough of these food. Food markets should add these elements so that more people could get needed amount of these necessary matters.

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