Auditor: XYZ, a construction company, purchased 20 new trucks 3 years ago, and there is no record of any of those trucks being sold last year. Records indicate, however, that XYZ sold off all of its diesel-powered trucks last year. We can thus conclude that none of the 20 trucks purchased 3 years ago were diesel powered. Which one of the following is an assumption required by the auditor's reasoning? ['All of the trucks that XYZ sold last year were diesel powered.', 'None of the 20 trucks was sold before last year.', 'XYZ did not purchase any used trucks 3 years ago.', 'XYZ no longer owns any trucks that it purchased more than 3 years ago.']