A thin semicircular conducting the ring (PQR) of radius 'r' is falling with its plane vertical in a horizontal magnetic field B, as shown in figure. Teh potential difference developed across the ring when its speed is v, is:<br>

Question

( A ) $2rBv$ and $R$ is at higher potential
( B ) $\pi rBv$ and $R$ is at higher potential
( C ) $2rBv$ and $R$ is at higher potential
( D ) $Bv \pi r^2/2$ and $P$ is at higher potential