A group of 14 students plans to participate in a free-throw competition to see how long it takes them to make 30 free-throws. The dot plots show the data before and after training. Which statement is true?
a The mean time the students took to finish the 30 free-throws increased by about 3 seconds after training.
b The mean time the students took to finish the 30 free-throws decreased by about 3 seconds after training.
c The slight difference in the distributions indicates that the training did not significantly change the students' race times.
d The significant difference in the distributions indicates that the training did not significantly change the students' race times.
