In a plunge grinding operation, the workpiece is 10mm wide, the wheel is of 20cm diameter and 2 cm wide. The wheel speed is 3000 r.p.m and the table speed is 2.5m/min. If the depth of cut is 0.02mm, grain density is 250/cm2 and the grinding forces are 20N tangential and 38 N thrust, the specific energy is
(a) 75.4 
N mm2
(b) 7.54 104 N/ mm2
(c) 75 
N/ mm2
(d) 7.54 N/ mm2