Source: GMATPrep

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 nonsmoker. Clearly, therefore, 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?

A) Smokers are less likely than nonsmokers to have diets that are rich in vitamins and minerals

B) The lung function of smokers whose diet are rich in those vitamins and minerals is generally better than that of nonsmokers with comparable diets

C) People whose diets are deficient in those vitamins and minerals do not typically have other health problems in addition to diminished lung function.

D) Stopping smoking will not typically improve lung function more than any diet changes can.

E) Smoking does not introduce into the body chemicals that prevent the helpful vitamins and minerals from entering the bloodstream

I could identify A, D and E to be contenders. Finally, I went with D. Can the instructors explain why is E the best answer. What is wrong with A and D ?

Thanks,
Sheetal

the first premise states, as fact, the following correlation:
higher vita/min in bloodstream <--> better lung function

that correlation is presented as an established fact, and so we are not allowed to question it. however, notice that the correlation is only asserted for vitamins and minerals in the bloodstream - i.e., they have to get into the bloodstream in the first place, not merely be ingested.

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notice also that the correlation is a FACT, regardless of whether the person in question is a smoker. in other words, if smokers get higher levels of the vitamins/minerals into their bloodstreams, then a fortiori they will also have better lung function.

the argument is therefore airtight, provided the smokers can get the vitamins and minerals into their bloodstreams to begin with. that's the only missing link here - all that's mentioned in the argument is intake, which is an entirely separate matter from absorption into the bloodstream.

choice e is correct because it takes care of the connection between intake and absorption, by establishing that the smokers' intake of vitamins and minerals will actually make it into the bloodstream.

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choice a is irrelevant; it speaks of vitamins and minerals in general, not the special vitamins and minerals mentioned in the passage. what's more, if it does anything it actually strengthens the conclusion (because adding vitamins/minerals to a vitamin- and mineral-poor diet will likely have greater consequences than will adding them to a diet that is already vitamin- and mineral-rich)!

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choice d is irrelevant, as cessation of smoking doesn't affect the issue (the effect of the vitamins/minerals) at all.

Thanks Ron for the great explanation.

On behalf of Ron, you're welcome!