In nature,abundance of $^1H_1$ and $^2H_1$ are 99.985 and 0.015 percent respectively and water obtained from it contains $H_2O$ and $D_2O$.The number of $D_2O$ molecules in 400mL water (if density is assumed to be 1.0g/mL) will be :

Question

$\begin{array}{1 1}(a)\;10^{21}&(b)\;2\times 10^{21}\\(c)\;3\times 10^{21}&(d)\;4\times 10^{21}\end{array}$