A、Control of natural hazards. B、Control of heavy traffic. C、Control of heart disease. D、Control of man-made hazards. D短文说，“在人类历史

Professors from Yale University agreed in a panel discussion tonight that the automobile was what one of them called "Public Health Enemy No. 1 in This Country". Besides polluting the air and overcrowding the cities, automobiles could cause heart disease "because we don’t walk anywhere any more," said Dr. H. P. Richard Weinerman, professor of medicine and public health. Dr. Weiner-man’s sharp accusation against the automobile came in a discussion of human environment on Yale Reports, a radio program broadcast by Station WTIC in Hartford, Connecticut. The program opened a three-part series on "Staying Alive". "For the first time in human history, the problem of man’s survival has to do with his control of man-made hazards," Dr. Weinerman said. "Before this, the problem had been the control of natural hazards."
16. What is the main idea of the passage?
17. Why could automobiles cause heart disease?
18. For the first time, what does the problem of man’s survival have to do with?