A 1-m-inner-diameter liquid-oxygen storage tank at a hospital keeps the liquid oxygen at 90 K. The tank consists of a 0.5-cm-thick aluminum (k = 170 W/m · K) shell whose exterior is covered with a 10-cm-thick layer of insulation (k = 0.02 W/m · K). The insulation is exposed to the ambient air at 20°C and the heat transfer coefficient on the exterior side of the insulation is 5 W/m²·K. The temperature of the exterior surface of the insulation is
(a) 13°C

(b) 9°C

(c) 2°C

(d) 23°C

(e) 212°C