The walls of a food storage facility are made of a 2-cm-thick layer of wood (k = 0.1 W/m · K) in contact with a 5-cm-thick layer of polyurethane foam (k = 0.03 W/m · K). If the temperature of the surface of the wood is 210°C and the temperature of the surface of the polyurethane foam is 20°C, the temperature of the surface where the two layers are in contact is
(a) 27°C

(b) 22°C

(c) 3°C

(d) 8°C

(e) 11°C