$A$ long straight rod of diameter $4 \ mm$ carries a steady current '$i$'. The current is uniformly distributed across its cross-section. The ratio of the magnetic fields at distances $1 \ mm$ and $4 \ mm$ from the axis of the rod is

  • A
    $1:2$
  • B
    $1:4$
  • C
    $4:1$
  • D
    $1:1$

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