A long straight wire of radius a carries a steady current $I$. The current uniformly distributed over its cross-section. The ratio of the magnetic fields $B$ and $B'$, at radial distance $\frac{a}{2}$ and $2a$ respectively, from the axis of the wire is :

Question

( A ) 1
( B ) 4
( C ) $\frac{1}{4}$
( D ) $\frac{1}{2}$