Although air pollution was previously thought to exist almost exclusiv

P1: air pollution was previously thought to exist almost exclusively in our nation’s cities
P2: the recent increase in the number of persons suffering from illnesses are attributed to excessive air pollution
Conclusion: nonurban areas are now affected.

pre-think: Increase in the number of reported cases attributed to air pollution => nonurban areas are now affected?

(A) The nation’s cities have seen a marked decrease in levels of air pollution. - Incorrect
This goes against the premise

(B) The nation has experienced a sharp decrease in the number of people moving out of its cities. - Incorrect
The premise #2 mentions that the number of people suffering has increased but does not mention anything about the area in which the increased number is coming from. Therefore even if this is true and fewer people are moving out of the city the overall number is still increasing and the argument still holds.

(C) Illnesses due to air pollution are among the least common causes of death to urban dwellers. - Incorrect
Out of scope. The argument is mainly concerned with illness related to air pollution

(D) Many illnesses previously thought unrelated to air pollution are now considered to be caused by it. - Correct
If this is true then the conclusion is wrong in saying that the nonurban areas are now affected. The people in the nonurban areas may have been affected a long time ago but their illness could have recently been attributed to air pollution

(E) As a result of the problems in urban areas, nonurban areas have passed strict pollution control measures. - Incorrect
Irrelevant. Was this done after the nonurban areas were affected? If true the conclusion will still hold. Moreover, the argument uses the increased number of cases as a basis to claim that nonurban areas are affected. This option does not explain anything with regards to the number

Hi VeritasKarishma GMATNinja

I have a doubt in Choice B:

If the number of people leaving the cities decreases, then we can say "increase in the number of persons suffering from illnesses attributed to excessive air pollution" can be due to the increased number of people staying in the cities.

Let's say, there were 500 people in the cities with the migration rate X. After migration the number of people is 400. However, if the migration rate is X/2, then the strength of city dwellers after migration would be 450, more than when the migration rate was X.
Since more people are now exposed to the polluted air in the city, this increased number of people in the city can be the reason for increase in the number of people suffering from illnesses attributed to excessive air pollution

Hence, it weakens the conclusion that the "nonurban areas are now affected"

Please help me in understanding where I am faltering.

Yes, I agree. I did park this option on the side after reading it. It wasn't perfect but it did make some sense. The reason I wasn't happy with it is that if people are staying put in cities, the number of cases shouldn't increase. Is there some cycle that lots of people come in and lots go out of urban areas routinely and hence they spend only limited time there? We don't know. What happens normally is that people move to urban areas looking for jobs hence there is usually an influx. So this option didn't seem very well thought out but nevertheless did lead to some weakening of the conclusion.

Of course, after reading (D) it was clear that (D) is the answer and (B) is just a bad option.