Evaluations for input

<image 1>The accompanying sketch shows the schematic arrangement for measuring the thermal conductivity by the guarded hot plate method. Two similar 1 cm thick specimens receive heat from a 6.5 cm by 6.5 cm guard heater. When the power dissipation by the wattmeter was 15 W, the thermocouples inserted at the hot and cold surfaces indicated temperatures as 325 K and 300 K. What is the thermal conductivity of the test specimen material?

(A) 0.81 W/m K

(B) 0.71 W/m K

(C) 0.61 W/m K

(D) 0.51 W/m K

Answer with the option's letter from the given choices directly. No punctuation.