The Inner Wall of a Furnace Is at a Temperature

The inner wall of a furnace is at a temperature of 700°C. The composite wall is made of two substances, 10 and ² 0 cm thick with thermal conductivities of 0.05 and 0.1 W.m^-1 .°C1 respectively. The ambient air is at 30°C and the heat transfer co-efficient between the outer surface of wall and air is 20 W.m ^-2 .°C^-1 . The rate of heat loss from the outer surface in W.m ^-2 is

A) 165.4
B) 167.5
C) 172.5
D) 175