The magnetic field normal to the plane of a wire coil of $n$ turns and radius $r$ carrying a current $i$ is measured on the axis of the coil at a small distance $h$ from the centre. By what fraction is this field smaller than the field at the centre?

• A
  $\frac{3}{2}\frac{h^2}{r^2}$
• B
  $\frac{2}{3}\frac{h^2}{r^2}$
• C
  $\frac{3}{2}\frac{r^2}{h^2}$
• D
  $\frac{2}{3}\frac{r^2}{h^2}$