Ferrous oxide has a cubic structure and each edge of the unit cell is

50 Å  . Assuming density of the oxide as 4.0g – cm ^-3     then the number of  Fe²⁺   and  O^2-  ions present in each unit cell will be

a. Four Fe²⁺ and four O^2-

b. Two  Fe²⁺  and four O^2-

c.  Four Fe²⁺    and  O^2-

d.Three Fe²⁺   and three  O^2-

Ans. a