A highly rigid cubical block A of small mass M and side L is fixed rigidly to another cubical block B of the same dimensions and 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 horizontal surface. A small force is applied perpendicular to one of the side face of A. After the force is withdrawn block A executes small oscillations the time period of which is given by

Question

\[\begin {array} {1 1} (a)\;2 \pi \sqrt {M \eta L} & \quad \\ (b)\;2 \pi \sqrt {\frac{M \eta}{L}} & \quad \\ (c)\;2 \pi \sqrt {\frac{M }{\eta L}} & \quad \\ (d)\;4 \pi \sqrt {\frac{M}{\eta L}}\end {array}\]