A mass $m$ is suspended from two springs of spring constants $k_{1}$ and $k_{2}$ as shown. 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) $2 \pi \sqrt{\frac{m k_{1} k_{2}}{\left(k_{1}+k_{2}\right)}}$
(d) $2 \pi \sqrt{\frac{m\left(k_{1}+k_{2}\right)}{\left(k_{1} k_{2}\right)}}$