Cigarette smoking, which is widely accepted to be the leading cause of

Cigarette smoking, which is widely accepted to be the leading cause of lung cancer, is less popular today than ever before. Therefore, we can expect the incidence of lung cancer to decline in the future.

Which of the following, if true, would most seriously weaken the argument above?

Premise: Cigarette is a leading cause of Lung cancer. NOW, fewer people smoke cigarettes.
Conclusion: As there is less cigarette smoking, we will expect a decline in lung cancer patients.

Pre-thinking: Usually in cause-effect questions, try to see whether the effect is only due to the given CAUSE (S).
There is no mention that Lung cancer is ONLY caused by cigarette smoking. It can be caused by various other reasons.

(A) A major cigarette manufacturer will soon introduce a new type of cigarette pack that contains fewer cigarettes than its other types of packs.
If anything, this strengthens the argument. Less cigarettes -> Less lung cancer DUE to cigarettes.

(B) Cigarette smoking is gaining in popularity compared to cigar smoking, which is also known to cause lung cancer.
But relative to past cigarette smoking, presently less people smoke cigarettes. So, we are looking for overall lung cancer patients. We are not comparing the lung cancer incidents caused by cigarette and by cigar smoking. Out of scope.

(C) People rarely develop lung cancer within a few years after they begin smoking cigarettes.
Gap here: nothing is said about the duration people smoke cigarettes. Few years [how many] -- we don't know.
Even if we ignore this gap, it would mean, cigarette or no cigarette -> it doesn't significantly contribute to cause cancer. Neither strengthen nor weaken.

(D) The nation that leads the world in exporting tobacco used in cigarettes is expected to place limits on the amount of tobacco it exports.
Less tobacco -> less cigarettes -> less cases of lung cancer

(E) An increasing number of people are developing lung cancer as a result of causes other than cigarette smoking.
BINGO! this is what we were looking for. This proves that LUNG cancer can be caused by other factors as well and INCREASING number of people are developing it. So, in the future, there will be increase in lung cancer patients.

IMO, E.
Also, Quit smoking