The rational is this: Less smokers should lead to less SIB fires, thus lead to less home fires, thus lead to less death in home fires. However this is not happening, why? Basically, it wants you to pick the one that cannot help answer this question. If an answer can break one link in this chain, then it is NOT our answer.
(A) says that SIB fires do not cause death, so less smokers has no impact on deaths in home fires.
(C) says that less smokers does not lead to less SIB fires because SIB fires are caused by heavy smokers who do not quit.
(D) says less SIB fires does not mean less home fires, since other types of home fires have increased
(E) says less home fires does not mean less death in home fires, since each fire now have more deaths because of home density
Only (B) does not help, since it says SIB fires likely cause death, therefore the chain should work, and so cannot explain the discrepensy.