[单选] 根据下列短文，回答题。　　Despite the fact that today viruses(病毒)are known to cause cancer in animals and in

A . viruses are known to cause cancer in animals
B . at present，human cancer is not believed to be contagious(传染的)
C . there are many known causes for the transformation of a normal cell to a cancer cell
D . results of experiments on plants and animals do not vary greatly from species to species

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