# A horizontal overhead power line is at a height of 4m from the ground and carries a current of 100 A from east to west. The magnetic field directly below it on the ground is $(\mu_0 = 4\pi\times 10^{-7} T m A^{-1})$
( A ) $5\times 10^{-6}$ T northward
( B ) $2.5\times 10^{-7}$ T southward
( C ) $2.5\times 10^{-7}$ T northward
( D ) $5\times 10^{-6}$ T southward