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 :

• A

  Zero

• B

  Bv$\mathrm{\pi }$r²/2 and P is at higher potential

• C

  $\mathrm{\pi }$rBv and R is at higher potential

• D

  2rBv and R is at higher potential