A question

I need help for this Logic Reasoning Question:

Opponents of allowing triple-trailer trucks to use the national highway system are wrong in claiming that these trucks are more dangerous than other commercial vehicles. In the western part of the country, in areas where triple-trailers are now permitted on some highways, for these vehicles the rate of road accident fatalities per mile of travel is lower than the national rate for other types of commercial vehicles. Clearly, triple-trailers are safer than other commercial vehicles.

Which one of the following, if true, most substantially weakens the argument?

(A) It takes two smaller semitrailers to haul as much weight as a single triple-trailer can.

(B) Highways in the sparsely populated West are much less heavily traveled and consequently are far safer than highways in the national system as a whole.

(C) Opponents of the triple-trailers also once opposed the shorter twin-trailers, which are now common on the nation's highways.

(D) In areas where the triple-trailers are permitted, drivers need a special license to operate them.

(E) For triple-trailers the rate of road accident fatalities per mile of travel was higher last year than in the two previous years.

I chose B, but the right answer is A, I still don't know why.

The right key must be B. A is definately impossible.

B supplies another reason, lower travelling rate, that calls into question the legitimacy of cited evidence, thus weakening the conclusion.

A  nothing from it can we acquire to use as evidence to deny that triple-trailer is safer than other vehicles.