When water, as in a pond or lake, is heated by warm air above it, it remains stable, does not move, and forms a warm layer of water on top of a cold layer. Consider a deep lake (k = 0.6 W/m · K, c_p = 4.179 kJ/kg · K) that is initially at a uniform temperature of 2°C and has its surface temperature suddenly increased to 20°C by a spring weather front. The temperature of the water 1 m below the surface 400 hours after this change is
(a) 2.1°C

(b) 4.2°C

(c) 6.3°C

(d) 8.4°C

(e) 10.2°C