The Spheres of different materials one with double the radius and one-fourth wall thickness of the other are filled with ice. If the time taken for complete melting of ice in large radius one is 25 minutes and that for smaller one is 16 minutes, the ratio of thermal conductivities of materials of large sphere to small sphere is:

Question

\[(a)\;4:5 \qquad (b)\;5:4 \qquad (c)\;25:8 \qquad (d)\;1:25\]