A company is evaluating a new reading program that is supposed to improve test scores. The box plots show the results of reading tests given before and after using the program. With the data, what is the most reasonable answer to the question: Does the program improve test scores?

A: No, because some students may have done worse on the post-test than on the pre-test.
B: No, because the low score of 20 and the high score of 90 did not change.
C: Yes, because the median test score increased 5 points.
D: Yes, because the same number of students scored above and below the median on each test.