Which is the incorrect statement? (a) Density decreases in case of crystals with Schottky defect. (b) NaCls is insulator silicon is semiconductor silver is conductor. quartz is piezoelectric crystal. (c ) Frenkel defect is favoured in those ionic compounds in which sizes of cation and anions are almost equal.(d) FeO0.98 has non-stoichiometric metal deficiency defect