The first of the two sample has 100 items with mean 15 and S.D= 3. If the whole group has 250 items with mean 15.6 and SD = $ \sqrt{13.44}$ the S.D of the second group is?

Question

$\begin {array} {1 1} (A)\;5 & \quad (B)\;4 \\ (C)\;6 & \quad (D)\;3.52 \end {array}$