premise: A train makes five trips around a loop through five stations—P, Q, R, S, and T, in that order—stopping at exactly three of the stations on each trip. The train must conform to the following conditions: The train stops at any given station on exactly three trips, but not on three consecutive trips. The train stops at any given station at least once in any two consecutive trips. Suppose that the train's scheduler mistakenly scheduled the train to make the following stops on the first three trips: Q, R, and S on the first trip; P, Q, and S on the second trip; and R, S, and T on the third trip. Which one of the following is a substitution that results in a schedule for those three trips under which the train violates no condition?
hypothesis: on the first trip, P for Q