A thin semicircular conducting ring, of radius R, in falling with its plane vertical in a uniform horizontal magnetic field of strength B. At the position MNQ the speed of the ring is V. The potential difference, developed across the ring, would than be
(1) 1/2 BVπR2 with M at higher potential
(2) πRBV , with Q at higher potential
(3) 2RBV, with Q at higher potential
(4) 2R2BV, with M at higher potential