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 After the force is withdrawn block \[A\] executes small oscillations.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 [IIT 1992]

• A.  \[2\pi \sqrt{\frac{M\eta }{L}}\]
• B.   \[2\pi \sqrt{\frac{L}{M\eta }}\]
• C. If assertion is true but reason is false.    
• D.    \[2\pi \sqrt{\frac{M}{\eta L}}\]