On a smooth inclined plane,a mass $M$ is attached between two massless springs of force constant $k$ each,as shown in the figure. The other ends of the springs are fixed to firm supports. The period of oscillation of the mass $M$ is

• A
  $2 \pi \sqrt{\frac{M}{2 k}}$
• B
  $2 \pi \sqrt{\frac{2 M}{k}}$
• C
  $2 \pi \sqrt{\frac{M g \sin \theta}{2 k}}$
• D
  $2 \pi \sqrt{\frac{2 M g}{k}}$