The velocity of wind at a wind turbine is measured to be 8 m/s. The blade span diameter is 12 m. The density of air is 1.2 kg/m³. If the horizontal force exerted by the wind on the supporting mast of the wind turbine is 1620 N, the efficiency of the wind turbine is

(a) 27.5% 

(b) 31.7% 

(c) 29.5% 

(d) 35.1% 

(e) 33.8%