[单项选择]In 1911, Americans smoked almost 10 billion cigarettes. Sixty years later this number (21) 700 billion. (22) this amazing increase, the demand for cigarettes in the United States is now (23) dropping. Since 1973, per capita cigarette (24) has decreased approximately I percent (25) . According to some estimates, 90 percent of all cigarette smokers would like to cut down or quit smoking (26) , but that, of course, is not easy to do and (27) do not succeed.
Smokers who try to " (28) the habit" may experience both physical and psychological withdrawal symptoms for several weeks. They may suffer, for example, (29) headaches, nausea, irritability, and an inability to concentrate. Some (30) , such as drowsiness and craving (a strong desire for a cigarette), get even (31) after the first ten days. Most people continue to crave cigarettes for at least a month, and approximately one-fifth continue to (32) th