The magnetic field at the centre of a circular coil of radius $r$,due to current $I$ flowing through it,is $B$. The magnetic field at a point along the axis at a distance $r/2$ from the centre is

  • A
    $B / 2$
  • B
    $2 B$
  • C
    $\left(\frac{2}{\sqrt{5}}\right)^{3} B$
  • D
    $\left(\frac{2}{\sqrt{3}}\right)^{3} B$

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