Earth has an excess of 6.0×10⁵ electrons on each square centimeter of surface. Estimate the electric charge of Earth in coulombs, considering Earth's radius as 6400 km.
a) 4.02×10¹⁵ C
b) 9.6×10¹⁰ C
c) 3.84×10¹¹ C
d) 6.67×10⁻¹¹ C