$XPQY$ is a vertical smooth long loop having a total resistance $R$, where $PX$ is parallel to $QY$ and the separation between them is $l$. $A$ constant magnetic field $B$ perpendicular to the plane of the loop exists in the entire space. $A$ rod $CD$ of length $L$ $(L > l)$ and mass $m$ is made to slide down from rest under gravity as shown in the figure. The terminal speed acquired by the rod is . . . . . . $m/s$. ($g$ = acceleration due to gravity)

• A
  $ \frac{2mgR}{B^{2}l^{2}} $
• B
  $ \frac{8mgR}{B^{2}l^{2}} $
• C
  $ \frac{2mgR}{B^{2}L^{2}} $
• D
  $ \frac{mgR}{B^{2}l^{2}} $