A brass cube of side a and density s is floating in mercury of densityr. If the cube is displaced a bit vertically, it executes S.H.M. Its time period will be

• A.            \[2\pi \sqrt{\frac{\sigma \,a}{\rho \,g}}\]                    
• B.            \[2\pi \sqrt{\frac{\rho \,a}{\sigma \,g}}\]                    
• C.            \[2\pi \sqrt{\frac{\rho \,g}{\sigma \,a}}\]                    
• D.            \[2\pi \sqrt{\frac{\sigma \,g}{\rho \,a}}\]