An English class took a field trip to a writing conference on a charter bus. They noticed that, even though they traveled in rain on the way back, the trip back took a shorter amount of time than the trip to the conference did. Which of the following is a valid conclusion? Assume the bus traveled the same route both directions and did not stop.
A.
The rain caused a decrease in travel time.

B.
There is a positive correlation between rain and travel time.

C.
An increase in the speed of the bus caused a decrease in travel time.

D.
There is no correlation between bus speed and travel time.