A current vs time graph of the current passing through a solenoid is shown in the fig. For which time in the back electromotive force (u) a maximum? If the back emf at t=3s is e, find the back emf at t = 7s. OA, AB and BC are straight line segments.

Question

(a) 3e
(b) -3e
(c) $\pm {3e}$
(d) none of these