On a trip, a cyclist averaged 12 miles per hour for the first 18 miles and 10 miles per hour for the remaining 18 miles. If the cyclist returned immediately via the same route and took a total of 7.3 hours for the round trip, what was the average speed (in miles per hour) for the return trip?  Choose the correct choice from the following answers:
A) 8.4 B) 8.6 C) 8.8 D) 9 E) 9.2

Rationale: "the time to go 36 miles was 18 / 12 + 18 / 10 = 1.5 + 1.8 = 3.3 hours . the average speed for the return trip was 36 miles / 4 hours = 9 mph . the answer is d ."
Answer: D