( A ) $NaCl(s)$ is insulator, silicon is semiconductor, silver is conductor, quartz is piezo electric crystal
( B ) $FeO_{0.98}$ has non stoichiometric metal deficiency defect
( C ) Frenkel defect is favoured in those ionic compounds in which sizes of cation and anions are almost equal
( D ) Density decreases in case of crystals with Schottky's defect
