What is the strength of an electric field that will balance the weight of a 7.5 g plastic sphere that has been charged to -7.4 nC?
a) 9.87 × 10³ N/C
b) 1.15 × 10⁴ N/C
c) -9.87 × 10³ N/C
d) -1.15 × 10⁴ N/C