What is the force of attraction in Newtons between a cation with a valence of 4 and an anion with a valence of 4, the centers of which are separated by a distance of 5.1 nm?
a) 2.56 x 10⁻9 N
b) 6.4 x 10⁻9 N
c) 1.28 x 10⁻8 N
d) 3.2 x 10⁻8 N