Cars X and Y leave City A at the same time and travel the same route to ... ...

Cars X and Y leave City A at the same time and travel the same route to City B. Car X takes 30 minutes to complete the trip and car Y takes 20 minutes. Which of the following must be true?

I. The average miles per hour at which car X traveled was greater than the average miles per hour at which car Y traveled.
II. The distance between the cities is 30 miles.
III. The average miles per hour at which car Y traveled was greater than the average miles per hour at which car X traveled.


(A) I only
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Nova Math - Problem Set O: Averages