Consider a long straight wire of a circular cross-section (radius a) carrying a steady current I. The current is uniformly distributed across this cross-section. The distances from the centre of the wire's cross-section at which the magnetic field [inside the wire, outside the wire] is half of the maximum possible magnetic field, any where due to the wire, will be

A

$$[a/4,3a/2]$$

B

$$[a/4,2a]$$

C

$$[a/2,2a]$$

D

$$[a/2,3a]$$