单项选择题

The possibility that a virus causes human cancer is indicated by （）

A. the fact that viruses have been known to mutate
B. the fact that a cancer-immune individual may lose his immunity
C. the fact that production of human cancer cells might be due to a genetic factor
D. the fact that man is host to many viruses

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单项选择题

Cancer should be considered to be a biological problem rather than a medical one because （）

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

单项选择题

What is main purpose of the passage（）

A. To outline contrasting types of economy.
B. To explain the science of economics.
C. To argue for the superiority of one economics.
D. To compare barter and money exchange markets.