What is the critical radius of insulation (cms) for a metallic cylinder, if the convective heat transfer co-efficient with the ambient atmosphere is 5 W/m2. °K? Thermal conductivity of metal and insulation material are 40 and 0.1 W/m.°K respectively.

A. 2

B. 8

C. 10

D. 40

Please do not use chat terms. Example: avoid using "grt" instead of "great".