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 speed $\nu$, then the emf induced in the circuit in terms of $B, l$ and $\nu$, where $l$ is the width of each tube, will be : <br>

Question

( A ) $-B l \nu$
( B ) $2 B l \nu$
( C ) zero
( D ) $-B l \nu$