A small dc motors operating at 200 V draws a current of 3 A at its full speed of 2500 r.p.m. If the resistance of the armature is $7\Omega$, the back e.m.f. of the motor is

149 V     159 V  169 V  179 V

179 V

$ \xi = V - IR = 200 - 3\times 7 = 179V$