A thin semicircular conducting ring (PQR) of radius 'r' is falling with its plane vertical in a horizontal magnetic field B, as shown in figure. The potential difference developed across the ring when its speed is v, is : XX X XBX X XX XXX X X X R X X X X X X X (a) Zero (b) Bvr /2 and P is at higher potnetial (c) rv and (d) 2rBv and R is at higher potential Ris at higher potnetial