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单项选择题The best title for this passage is （）

A. New Light on the Cause of Cancer
B. The Newest Theory on the Nature of Viruses
C. Viruses and Cancer
D. On the Nature of Life

单项选择题Some years ago safety rules （）

A. were put forward due to scientists’ recommendations
B. came into being as a result of the workers’ demand
C. were introduced because quite a number of people were killed or seriously injured
D. were effective enough to protect workers and customers