Please Explain Question No 14

14.  A rod AB slides on a V shaped wire with speed v as shown, such that at any time OA = OB = l. Magnetic field in the region is perpendicular downwards and has strength B, induced emf in the rod is
 

$$\begin{gathered} \left(1\right) Zero \left(2\right) Bvl \\ \left(3\right) \frac{\sqrt{3} Bvl}{2} \left(4\right) \frac{Bvl}{2} \end{gathered}$$