The pycnometric density of sodium chloride crystal is $2.165\times 10^3kg m^{-3}$ while its X-ray density is $2.178\times 10^3kg m^{-3}$.The fraction of the unoccupied sites in sodium chloride crystal is

Question

$\begin{array}{1 1}5.96\times 10^{-3}\\4.96\times 10^{-3}\\3.96\times 10^{-3}\\2.96\times 10^{-3}\end{array} $