单项选择题

Until the twentieth century cigarettes were not an important threat to public health. Men used tobacco mainly in the form of cigars, chewing tobacco, pipe tobacco, and snuff(鼻烟). Most women did not use tobacco at all.
The cigarettes industry began in the 1870s with the development of the cigarette manufacturing machine. This made it possible to produce great numbers of cigarettes very quickly, and it reduced the price.
Today cigarette smoking is a widespread habit. About forty-three percent of adult men and thirty-one percent of adult women in the United States smoke cigarettes regularly. It is encouraging to note, however, that millions of people have given up the smoking habit. Seventy-five percent of the male population and forty-six percent of the female population have smoked cigarettes at some time during their lives, but twenty-six percent of these men and eleven percent of women have stopped smoking. The number of persons who have given up smoking is increasing.
Men as a group smoke more than women. Among both men and women .the age group with the highest proportion of smokers is the age group 24-44.
Income, education and occupation’ all play a part in determining a person’s smoking habits. City people smoke more than people living on farms. Well-educated men with high incomes are less likely to smoke cigarettes than men with fewer years of schooling and lower incomes. On the other hand, if a well-educated man with a higher income smokes at all, he is likely to smoke more packs of cigarettes per day.
The situation is somewhat different for women. There are slightly more smokers among women with higher family incomes and higher education than among the lower income and lower educational groups. These more highly educated women tend to smoke more heavily.
Among teenagers there are fewer smokers from upper-in-come, well-educated families, and fewer from families living in farm areas. High school students who are preparing for college are less likely to smoke than those who do not plan to continue their education after high school. Children are most likely to start smoking if one or both of their parents smoke.

All of the following factors were dangerous for people’s health be-fore the twentieth century except（）

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