Four massless springs whose force constants are 2k, 2k, k and 2k respectively are attached to a mass M kept on a frictionless plane (as shown in Fig.) If the mass M is displaced in the horizontal direction, then the frequency of the system:
 

• A

  $\frac{1}{2\mathrm{\pi }}\sqrt{\frac{\mathrm{k}}{4\mathrm{M}}}$

• B

  $\frac{1}{2\mathrm{\pi }}\sqrt{\frac{4\mathrm{k}}{\mathrm{M}}}$

• C

  $\frac{1}{2\mathrm{\pi }}\sqrt{\frac{\mathrm{k}}{7\mathrm{M}}}$

• D

  $\frac{1}{2\mathrm{\pi }}\sqrt{\frac{7\mathrm{k}}{\mathrm{M}}}$