Let us suppose that the Earth (radius 6400 km) has a net charge equivalent to one electron per square metre of its surface area. The Earth's potential (in volt) at the surface will be:
a) 6.38 × 10⁶ V
b) 5.35 × 10⁶ V
c) 7.21 × 10⁶ V
d) 4.98 × 10⁶ V