Select the correct statements among the following

a) crystalline silica is used in photovoltaic cells

b) AgBr will show both Schottky and Frenkel defects

c) Among metals the best conductor is silver

d) NaCl crystal when heated in sodium vapours develop pink colour

amorphous  silica is used in photovoltaic cells

Sodium chloride on heating with sodium vapours acquires yellow colour because sodium chloride crystal suffers metal excess defect with sodium vapours on heated condition. Due to electronic transition at the excited state of sodium atom, it appears yellow.