At a primate reserve, the mean age of all the male primates is 15 years, and the mean age of all female primates is 19 years. Which of the following must be true about the mean age $m$ of all the combined group of male and female primates of the primate reserve ?

Question

$\begin{array}{1 1} (a)\; m=17\\(b)\; m>17 \\ (c)\;m<17 \\ (d)\; 15 < m <19 \end{array}$</span>