The temperature of a gas flowing in a long duct as measured by thermocouple (having an emissivity of 0.5) is 800 K. The internal wall surface of the duct is at a temperature of 500 K. The convective heat transfer coefficient between the gas and the tip of the thermocouple is 100 W^-1 . K .^-1 The actual gas temperature is actually . . . . . . . . K.

A. 400

B. 500

C. 820

D. 900

Answer: Option D