For two data sets each of size 5, the variances are given to be 4 and 5 and the corresponding means are given to be 2 and 4 respectively. The variance of the combined data set is

Question

$\begin {array} {1 1} (A)\;\large\frac{5}{2} & \quad (B)\;\large\frac{11}{2} \\ (C)\;6 & \quad (D)\;\large\frac{13}{2} \end {array}$