9. Which of the following statements about the structures of metals and ionic solids is (are) correct? I. Atoms in a solid consisting of only one element would have six nearest neighbors, if the crystal structure is a simple cubic array. II. There are four tetrahedral holes in a cubic closest packed unit cell. III. If the metal with an atomic radius of r has a body-centered cubic structure, the edge length of the unit cell will be . IV. For a simple cubic array, the radius of an interior sphere (cubic hole) in terms of the radius (r) of a sphere in the array is . V. Cobalt fluoride crystallizes in a cubic closest packed array of fluoride ions with the cobalt ions filling one-half of the octahedral holes, and thus the charge of cobalt ions is t3 in this compound. (A)III (B)II and V (C)III and IV (D)I, III, and IV (E)I, IV, and V