An air-cored solenoid with length 30cm, area of cross section $25 cm^2$ and number of turns 500, carries a current of 2.5A. The current is suddenly switched off in a brief time of $10^{-3}S.$ How much is the average back emf induced across the ends of the open switch in the circuit ? Ignore the variation in magnetic field near the ends of the solenoid ?

Question

(A) 65 V
(B) 0.65 V
(C) 6.5 V
(D) 650 V