The higher the level of certain vitamins and minerals in the bloodstream, the better a person' s lung function, as measured by the amount of air the person can expel in one second. The lung function of smokers is significantly worse, on average, than that of nonsmokers. Clearly, ttherefore, one way for smokers to improve their lung function is for them to increase their intake of foods that are rich in these helpful vitamins and minerals. Which of the following is an assumption on which this argument depends? ['Smokers are less likely than nonsmokers to have diets that are rich in vitamins and minerals.', 'Smoking does not introduce into the body chemicals that prevent the helpful vitamins and minerals from entering the bloodstream.', 'People whose diets are deficient in those vitamins and minerals do not typically have other health problems in addition to diminished lung function.', 'Stopping smoking will not typically improve lung function more than any diet changes can.']