Air at 10°C enters an 18-m-long rectangular duct of cross section 0.15 m × 0.20 m at a velocity of 4.5 m/s. The duct is subjected to uniform radiation heating throughout the surface at a rate of 400 W/m². The wall temperature at the exit of the duct is
(a) 58.88C

(b) 61.98C

(c) 64.68C

(d) 69.18C

(e) 75.58C

(For air, use k = 0.02551 W/m · K, Pr = 0.7296, v = 1.562 × 10^-5 m²/s, c_p = 1007 J/kg · K, ρ = 1.184 kg/m³.)