D

Americans smoke six thousand million cigarettes every year, that is, every person smokes about 4,195 cigarettes a year in the country of 18 years of age or more. It has been calculated that 51% of American men smoke while 34% of American women do so. Since 1939, scientific studies have shown that smoking does great harm to one’s health and it will shorten one’s life. Cigarette smoking is believed, by most research workers in the field, a very important cause in the development of the cancer of the lungs and the cancer of the throat and is believed to have much to do with some other kinds of cancers. Cigarette smokers suffer from the illness of the heart more often than those who don’t smoke. But strange, yet true, women are thought to be less affected, because when women smoke, they usually don’t breathe in the smoke so deeply. Most of doctors and researchers say, "Give up smoking. If you don’t smoke--don’t start!" Filters (过滤嘴) are required now to make smoking a bit safer, but they can only reduce, not get rid of, its poison. But there are still a number of doctors and research workers who are less sure of the effect of cigarette smoking on health. They believe that the cancers of the lungs, the throat and so on may also be caused by air pollution, or chemical poison that is now being used by farmers in large quantities to destroy plant pests and small animals.

It is believed by many that the cancers of the lungs and throat are caused ______ by smoking.