what will be the critical thickness of insulation if convective HT coefficient with ambient atmosphere is given?

Question

 A metal rod of 2cm diameter has a conductivity of 40 W/m K, which is to be insulated with an insulating material of conductivity of 0.1 W/m K. If the convective heat transfer coefficient with the ambient atmosphere is 5 W/m² K, the critical thickness of insulation will be

                                                (IES-03)

a. 1 cm                         b. 2 cm

c. 7 cm                         d. 8 cm