Which of the following expressions represents the density of an FCC crystal structure in terms of the lattice parameter a and the atomic radius r?
a) (4 * atomic mass) / (a³ * Avogadro's number)
b) (2 * atomic mass) / (a³ * Avogadro's number)
c) (4 * atomic mass) / (3 * pi * r³)
d) (2 * atomic mass) / (3 * pi * r³)