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 i
A. A lack of zinc in the body may slow recovery from a medical operatio.
B. So far no proper way has been found to treat the illness caused by a lack of zinc.
C. Some physical problems turn out to be related to a lack of zinc.
D. A lack of zinc in the body may be compensated by giving zinc sulphate.
已知有向图G的定义如下：
G=(V,E)
V={a,b,c,d,e}
E={＜a,b＞，＜a,c＞，＜b,c＞，＜b,d＞，＜c,d＞，＜e,c＞，＜e,d＞)
(1)画出G的图形；
(2)写出G的全部拓扑序列。
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