Which is the incorrect statement?

- (A) is correct: $\text{FeO}$ is usually non-stoichiometric because some ${\text{Fe}}^{2+}$ are replaced by ${\text{Fe}}^{3+}$.(B) is correct: Schottky defect involves missing ions, reducing density.(C) is correct: $\text{NaCl}$ solid is an insulator; silver is a metal; silicon is a semi-metal.(D) is incorrect: Frenkel defect is favored when there is a large difference in the sizes of cations and anions (allowing the smaller ion to fit into interstitial sites). Schottky defect is favored when sizes are similar.