[单项选择]　　Questions 45 to 48 are based on the following passage: 　　Americans smoke six thousand million cigarettes every year (1970 figures). This is roughly the equivalent of 4,195 cigarettes a year for every person in the country of 18 years of age or more. It is estimated that 51% of American men smoke compared with 34% of American women. Since 1939, numerous scientific studies have been conducted to determine whether smoking is a health hazard. Research teams have conducted studies that show beyond all reasonable doubt that tobacco smoking, particularly cigarette smoking is associated with a shortened life expectancy. Cigarette smoking is believed by most research workers in this field to be an important factor in the development of cancer of the lungs and cancer of the throat. Male cigarette smokers have a higher death rate from heart disease than non-smoking males. (Female smokers are thought to be less affected because they do not breathe in the smoke so deeply.) The majority of docto
A. reduce nervous stress
B. causes lung and throat cancer
C. make life shorter but exciting
D. broaden the blood vessels