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

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

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