Passage Summary: A company sold all its diesel trucks last year but didn't sell any of the 20 trucks it bought three years ago, so those 20 trucks couldn't have been diesel.

Conclusion: None of the 20 trucks purchased three years ago were diesel-powered.

Reasoning: All diesel trucks were sold last year, but none of the 20 trucks from three years ago were sold last year.

Analysis: This argument relies on the assumption that the 20 trucks were still in the company's possession at the start of last year. If those trucks had already been sold or destroyed two years ago, the fact that they weren't sold *last* year wouldn't prove they weren't diesel. To test a Necessary Assumption, use the Negation Test: if we assume the company *didn't* still own those trucks at the start of last year, the whole argument falls apart. Look for an answer that ensures those 20 trucks were actually part of the fleet when the 'sell-off' occurred.