$A$ highly rigid cubical block $A$ of small mass $M$ and side $L$ is fixed rigidly onto another cubical block $B$ of the same dimensions and of low modulus of rigidity $\eta$ such that the lower face of $A$ completely covers the upper face of $B$. The lower face of $B$ is rigidly held on a horizontal surface. $A$ small force $F$ is applied perpendicular to one of the side faces of $A$. After the force is withdrawn,block $A$ executes small oscillations. The time period of which is given by:

• A
  $2\pi \sqrt {\frac{M\eta}{L}}$
• B
  $2\pi \sqrt {\frac{L}{M\eta}}$
• C
  $2\pi \sqrt {\frac{ML}{\eta}}$
• D
  $2\pi \sqrt {\frac{M}{\eta L}}$