A steel sphere of radius 0.1 m at 400 K is immersed in an oil at 300 K. If the centre of the sphere reaches 350 K in 20 minutes, how long will it take for a 0.05 m radius steel sphere to reach the same temperature (at the centre) under identical conditions ? Assume that the conductive heat transfer co-efficient is infinitely large.

• A. 5 minutes
• B. 10 minutes
• C. 20 minutes
• D. 40 minutes