The pyknometric density of sodium chloride crystal is `2.165xx10^(3)kg m^(-3)` while its `X` ray density is `2.178xx10^(3)kg m^(-3)` the fraction of unoccupied sites in `NaCl` crystal is
A. 5.96
B. `5.96xx10^(-2)`
C. `5.96xx10^(-4)`
D. `5.96xx10^(-3)`