$A$ long straight wire of radius $r$ carries a steady current $I$. The current is uniformly distributed over its cross-section. The ratio $\left(\frac{B}{B^1}\right)$ of the magnetic field $B$ and $B^1$ at radial distances $\frac{r}{2}$ and $3r$ respectively,from the axis of the wire is:

  • A
    $\frac{1}{2}$
  • B
    $\frac{3}{2}$
  • C
    $\frac{5}{2}$
  • D
    $\frac{7}{2}$

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