Ferrous oxide has a cubic structure and each edge of the unit cell is $5.0 \, \mathring{A}$. Assuming the density of the oxide is $4.0 \, g \, cm^{-3}$,the number of $Fe^{2+}$ and $O^{2-}$ ions present in each unit cell will be ($M_w$ of $FeO = 72$)

• A
  Four $Fe^{2+}$ and four $O^{2-}$
• B
  Two $Fe^{2+}$ and four $O^{2-}$
• C
  Four $Fe^{2+}$ and two $O^{2-}$
• D
  Three $Fe^{2+}$ and three $O^{2-}$