None of the 20 trucks was sold before
Why this is right
If we negate this, it's saying that "some of the 20 trucks were sold prior to last year". That opens up the door to the Objection scenario we were considering. It's possible that 3 years ago, they bought these 20 trucks, some of which were diesel, and then they sold those diesel trucks 2 years ago. The only reason the author is convinced that none of the 20 were diesel is because the company sold off all its diesel last year but none of the 20 were sold last year. Note: just because we say "none of those 20 trucks were sold last year" doesn't mean "the company still possessed all 20 of those trucks". The author is assuming it does. His argument works if we know that all 20 of those trucks are still possessed. That would guarantee that none of them are diesel, since all the diesel trucks this company possessed were sold last year. This is a weird example of a correct answer on Necessary Assumption that is basically a Sufficient Assumption. And when we negate this, it weakens by creating some doubt -- the negation makes it possible that one of these 20 was diesel and that it was sold before last year. The negation doesn't actually refute the original argument, but a negation does not need to "destroy the argument", as students often say, in order to be the correct answer. We can think, "Which answer, if negated, most weakens", and the winner of that will always be the correct answer on Necessary Assumption.
Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.