A long straight wire of circular cross section with radius 'a' carries steady current 'I' which is uniformly distributed across the cross-section. The magnetic field in the region $r < a$ and $r > a$ is represented by:

The correct answer is Option (4)

$\vec B=\frac{μ_0Ia}{2\pi R}$    $[a<r]$

$\vec B=\frac{μ_0I}{2\pi a}$    $[a>r]$