A mass $m$ is suspended from two springs of spring constant $k_{1}$ and $k_{2}$ as shown in Fig. 9.23. The time period of vertical oscillations of the mass will be
(A) $2 \pi \sqrt{\left(\frac{k_{1}+k_{2}}{m}\right)}$
(B) $2 \pi \sqrt{\frac{m}{\left(k_{1}+k_{2}\right)}}$
(C) $2 \pi \sqrt{\frac{m\left(k_{1} k_{2}\right)}{\left(k_{1}+k_{2}\right)}}$
(D) $2 \pi \sqrt{\frac{m\left(k_{1}+k_{2}\right)}{\left(k_{1} k_{2}\right)}}$