$A$ train has to negotiate a curve of radius $r \ m$. The distance between the rails is $\ell \ m$ and the outer rail is raised above the inner rail by a distance of $h \ m$. If the angle of banking is small,the safety speed limit on this banked track is:

• A
  $\sqrt{rg \left(\frac{h}{\ell}\right)}$
• B
  $rg \frac{h}{\ell}$
• C
  $\frac{(\frac{h}{\ell})^2}{rg}$
• D
  $(rg \frac{h}{\ell})^2$