A long straight wire of radius $a$ carries a steady current $I.$ The current is uniformly distributed over its cross-section. The ratio of  the magnetic fields $B$ and $B',$ at radial distances $\frac{a}{2}$ and $2a$ respectively, from the axis of the wire is 
  • A$\frac{1}{2}$
  • B$1$
  • C$4$
  • D$\;\frac{1}{4}$
NEET 2016, Medium
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