Two sets each of $20$ observations , have the same standard deviation $5$. The first set has a mean $17$ and the second a mean $22$. Determine the standard deviation of the set obtained by combining the given two sets.

Question

$\begin{array}{1 1}(A)\;0.99\\(B)\;5.59\\(C)\;8\\(D)\;1.25\end{array} $