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 $$V$$, then the emf induced in the circuit in terms of $$B$$, $$\ell$$ and $$V$$ where $$\ell$$ is the width of each tube will be

A

$$B \ell V$$

B

$$-B \ell V$$

C

zero

D

$$2 B \ell V$$