单项选择题

The main topic of the passage is （）

A. conditions in the work place
B. the freedom of industries in the past
C. changes in industrial production
D. the safety and health of workers and customers

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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 tn species

单项选择题

As pointed out at the beginning of the passage, people used to think that air pollution （）

A. cause widespread damage in the countryside
B. affected the entire eastern half of the United States
C. had damaged effect on health
D. existed merely in urban and industries areas