Q: A sphere of solid material of specific gravity 8 has a concentric spherical cavity and just sinks in water. The ratio of radius of cavity to that of outer radius of the sphere must be

(a) $ \displaystyle \frac{7^{1/3}}{2} $

(b) $ \displaystyle \frac{5^{1/3}}{2} $

(c) $ \displaystyle \frac{9^{1/3}}{2} $

(d) $ \displaystyle \frac{3^{1/3}}{2} $

Ans: (a)