Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

Problem: a car traveled from san diego to san francisco at an average speed of 54 miles per hour. if the journey back took twice as long, what was the average speed of the trip ?
Options: a. 24., b. 32., c. 36., d. 42., e. 44.
c