单项选择题

Zinc(锌) is one of about 20 elements necessary for good health. It is present in the body in a very small amount, but it .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 substances were added to their food. One of these substances was zinc. Dr. Powys later tested the zinc treatment on an American airman recovering form medical operations. The airman who received zinc sulphate (硫酸盐) recovered in one half the usual time. Other doctors studied persons who had number of strange disorders. They treated these disorders successfully by giving the patients zinc sulphate.
The amount of zinc in the body’ can found by examining the 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 foods, Food makers should add these elements so that more people could get needed amounts of these essential substances.

Some health experts say food makers should add some elements（）people won't lack these essential substances.

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