13. One ionic crystal structure contains two ions, . ions occupy the face-centered cubic lattice points, and ions occupy all the tetrahedral holes. Which statement is invalid? (A) The ionic radius of is larger than the ionic radius . (B) The coordination numbers of and F- are 12 and 8 respectively. (C) The relation between the edge length a of the unit cell and is equivalent to a * √3/4 = . (D) There are 4 ions and 8 F- ions in a unit cell. (E) The empirical formula of the ionic crystal structure is CaF2.