Total Volume of atoms present in a face centered cubic unit cell of a metal is (r is atomic radius)

Question

$(a)\;\large\frac{16}{3}\pi r^3\qquad(b)\;\large\frac{20}{3}\pi r^3\qquad(c)\;\large\frac{24}{3}\pi r^3\qquad(d)\;\large\frac{12}{3}\pi r^3$