Which one of the following expressions is for calculating the atomic packing factor in a rock salt ionic crystal (ra and rb are the radius of the cation and the anion, respectively)
a. 4π/3 (RA+RB)³/[4(RA+RB)/√2]³ x 4
b. 4π/3 RA³+RB³/[4(RA+RB)/√2]³ x 4
c. 4π/3 RA³+RB³/[2(RA+RB)/√2]³ x 4
d. 4π/3 RA³+RB³/[2(RA+RB)]³ x 4
e. 4π/3 (RA+RB)³/[2(RA+RB)]³ x 4