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 Press: Set L - Averages

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