The horizontal component of the Earth's magnetic field at a place is uniform and its direction is from south to north. $A$ high current flows through a horizontal power line at this place from west to east. Consider two points $A$ and $B$ at equal distances from the wire,respectively above and below it. Giving reasons,explain where the magnetic field is stronger: at $A$ or at $B$.

(B) According to the Right-Hand Thumb Rule,the direction of the magnetic field produced by the current at point $A$ (above the wire) is from north to south.
The direction of the magnetic field produced by the current at point $B$ (below the wire) is from south to north.
The Earth's horizontal magnetic field is directed from south to north.
At point $A$,the Earth's magnetic field and the magnetic field due to the current are in opposite directions,so the net magnetic field is the difference between the two.
At point $B$,the Earth's magnetic field and the magnetic field due to the current are in the same direction,so the net magnetic field is the sum of the two.
Therefore,the total magnetic field is stronger at point $B$.