A long copper rod of diameter 2.0 cm is initially at a uniform temperature of 100°C. It is now exposed to an air stream at 18°C with a heat transfer coefficient of 200 W/m²·K. How long would it take for the copper rod to cool to an average temperature of 25°C? (Express the answers in seconds.) (Thermal properties of copper: k = 401 W/m·°C, rho = 8933 kg/m³, cp = 0.385 kJ/kg·°C.)
a) 10956 seconds
b) 11803 seconds
c) 12547 seconds
d) 13291 seconds
