$A$ wire of mass $m$ and length $l$ can slide freely on a pair of smooth,vertical rails (figure). $A$ magnetic field $B$ exists in the region in the direction perpendicular to the plane of the rails. The rails are connected at the top end by a capacitor of capacitance $C$. The acceleration of the wire,neglecting any electric resistance,is:

• A
  $\frac{mg}{m + CB^2l^2}$
• B
  $\frac{2mg}{m + CB^2l^2}$
• C
  $\frac{mg}{CB^2l^2}$
• D
  $\frac{mg}{2(m + CB^2l^2)}$