A horizontal overhead powerline is at a height of 4m from the ground and carries a current of 100A from east to west. The magnetic field directly below it on the ground is
(μ0 = 4π × 10–7TmA–1 )
(a) 2.5 × 10–7T northward
(b) 2.5 × 10–7T southward
(c) 5 × 10–6T northward
(d) 5 × 10–6T southward.