The dot plots represent the distributions of values in data sets R and S.

Data Set R has dots at 5, 6, 7, 8, 9.
Data Set S has dots at 5, 6, 7, 8, 8.

Which of the following statements must be true?

I. The mean of data set R is equal to the median of data set R.

II. The mean of data set S is less than the median of data set S.