4. Both ZnS and CaF₂ have a face-centered cubic unit cell where the S²⁻ and Ca²⁺ ions are closest packed in each structure. Which of the following is true?
(A) There are 4 tetrahedral holes empty in each structure.
(B) In both compounds, one-half of the tetrahedral holes are filled.
(C) In both compounds, all the tetrahedral holes are filled.
(D) There are 8 Zn²⁺ ions and 4 F⁻ ions in the unit cell.
(E) In ZnS, one-half of the tetrahedral holes are filled by Zn²⁺ ions, whereas in CaF₂ all the tetrahedral holes are filled with F⁻ ions.