A current distributed uniformly over its cross-section. The ratio of the magnetic fields due to the wire at distance a, 3a, and 2a, respectively, from the axis of the wire is:
(a) 1 : 3 : 2
(b) 1 : 9 : 4
(c) 1 : 1 : 2
(d) 1 : 1 : 1