A school records the number of books read by students in a month. The data consists of $130$ values. Specifically, the values $1,2,\dots ,15$ each appear $8$ times; the values $16,17$ each appear $4$ times; and the value $18$ appears $2$ times. Let $\mu$ be the mean, $M$ be the median, and $d$ be the median of the modes of this data set. Which of the following statements is true?