Two conducting rings of radii `r` and `2r` move in apposite directions with velocities `2 upsilon` and `upsilon` respectively on a conducting surface `S`. There is a uniform magnetic field of magnitude `B` perpendicular to the plane of the rings. The potential difference between the highest points of the two rings is
A. zero
B. `2rvB`
C. `4rvB`
D. `8rvB`