Zinc oxide $(ZnO)$ adopts the crystal structure shown below. The red spheres are $O^{2-}$ anions and the grey spheres are $Z{n}^{2+}$ cations. How many $Z{n}^{2+}$ ions are there per ZnO unit cell?

$Z{n}^{2+}$ ions per ZnO unit cell: $2].$