From 392mg of $H_2SO_4,1.204\times 10^{21}$ molecules are removed.How many moles of $H_2SO_4$ are left?

Question

$\begin{array}{1 1}(a)\;2.0\times 10^{-3}& (b)\;1.2\times 10^{-3} \\(c)\;4.0\times 10^{-3}& (d)\;1.5\times 10^{-3}\end{array}$