rebecw

Answer choice D: "ticketed more than once" does not ensure "a more representative sampling." Rather it states that there were repeated offenders on the highway.

Assumption question is different from strengthening question in that the former needs a statement which is necessary for the conclusion to be drawn while the latter simply shed more favorable light on the conclusion.

rebecw

When you negate A, you get "Drivers who equip their vehicles with radar detectors are more likely to be ticketed for exceeding the speed limit than are drivers who do not."

And this statement is consistent with one of the premise of the argument, therefore the argument still holds. IF you follow the logic chain of the author and adding the negated choice A), the argument is valid because that's what the author claims!

Whether or not the author's conclusion is right or wrong is not our concern for the assumption-type question. We have to treat the author's statements as correct. Since when you negate A, you do not weaken or refute the author's argument, the argument still holds. Therefore, A) is not the necessary assumption.