Select right expression for determinig packing fraction (P.F.) of NaCl unit cell (assume ideal ) , if ions along an edge diagonal are absent :
A. `((13)/(2)pir^(3)+(13)/(3)pir^(3))/(8(r_(+)-r_(-))^(3))`
B. `((13)/(3)pir_(-)^(3)+(4)/(3)pir_(+)^(3))/(8(r_(+)r_(-))^(3))`
C. `((16)/(3)pir_(-)^(3)+(13)/(3)pir_(+)^(3))/(8(r_(+)+r_(-))^(3))`
D. `((4)/(3)pir_(-)^(3)+(13)/(3)pir_(+)^(3))/(8(r_(+)+r_(-))^(3))`