A solid sphere of radius r made of a soft material of bulk modulus K is surrounded by a liquid in a cylindrical container. A massless piston of area a floats on the surface of the liquid, covering entire cross section of cylindrical container. When a mass m is placed on the surface of the piston to compress the liquid, the fractional decrement in the radius of the sphere $(\frac{dr}{r})$ is

Question

( A ) $\frac{ka}{3mg}$
( B ) $\frac{ka}{mg}$
( C ) $\frac{mg}{ka}$
( D ) $\frac{mg}{3ka}$