One conducting U tube can slide inside another as shown in figure, maintaining electrical contacts between the tubes. The magnetic field B is perpendicular to the plane of the figure. If each tube moves towards the other at a constant sped v, then the emf induced in the circuit in terms of B, l and v, where I is the width of each tube, will be

• A

  Zero

• B

  2Blv

• C

  Blv

• D

  $-Blv$