Q13. A long straight wire of radius a carries a steady current I. The current is uniformly distributed across its cross section. The ratio of the magnetic field at $\frac { a } { 2 }$ and $2 a$ from axis of the wire is :\\
(1) $1 : 4$\\
(2) $1 : 1$\\
(3) $3 : 4$\\
(4) $4 : 1$