$0.5\;g$ of fluming $H_2SO_4$(oleum) is diluted with water. This solution is completely neutralised by $26.7\;mL$ of $0.4\;N\;NaOH$. The percentage of free $SO_3$ in the sample is

Question

$(a)\;30.6 \% \\ (b)\;40.6\% \\(c)\;20.6 \% \\(d)\;50\%$