D vs. C

D. Is D right because if none of the 20 trucks purchased two years ago were sold before last year then it ensures that the company still has the 20 trucks it purchased two years ago, thus maintaining the conclusion that non of the trucks were diesel? I think I understand why C is wrong.

@tomgbean, (C) is incorrect because only the 20 trucks in question are relevant. Any information about trucks that the company has bought or already owns does not alter the conclusion. (D) is correct because we know the company purchased the 20 trucks in question 3 years ago and did not sell them last year, but we are not told anything about the first 2 years of ownership. Therefore, it is possible that the trucks were diesel and were sold during the second year they were with the company. This would present a scenario that diverges from the passage's conclusion, so (D) is a necessary assumption. Your explanation of (D) ensuring that the 20 trucks are still owned by the company and therefore non-diesel as concluded is correct. Does that help? Please let us know if you have any other questions!