A long straight wire with a circular cross-section having radius $R$, is carrying a steady current $I$. The current $I$ is uniformly distributed across this cross-section. Then the variation of magnetic field due to current $I$ with distance $r\left(r<R\right)$ from its centre will be 

• Correct answer
  A

  $B\propto r$
• B

  $B\propto \frac{1}{r}$
• C

  $B\propto r^2$
• D

  $B\propto \frac{1}{r^2}$

Hence, the correct answer is option: 'A'.