A 1-cm-diameter, 30-cm-long fin made of aluminum (k = 237 W/m · K) is attached to a surface at 80°C. The surface is exposed to ambient air at 22°C with a heat transfer coefficient of 11 W/m² · K. If the fin can be assumed to be very long, its efficiency is

(a) 0.60

(b) 0.67

(c) 0.72

(d) 0.77

(e) 0.88