The rate of heat flow through a composite wall of three layers of thickness 0.3 m, 0.2 m, 0.15 m and of corresponding thermal conductivities 1.2, 0.8 and 0.6 kJ/hr-m$$^\circ C$$ is 1280 kd/hr. If the surface area normal to the direction of heat flow is 1 sq.mtr. and inner surface temperature is $$1000^\circ C$$, then the interface temperature at the end of first layer will be

A

$$700^\circ C$$

B

$$680^\circ C$$

C

$$500^\circ C$$

D

$$360^\circ C$$