A horizontal overhead power line is at a height of 4m from the ground and carries a current of 100A from east to west. If µ0 = 4 π x 10-7 T-1 m-1, the magnetic field directly below it on the ground is
(A) 2.5 x 10-7 T northward
(B) 2.5 x 10-7 T southward
(C) 2.5 x 10-6 T northward
(D) 2.5×10-6 T southward
