Which of the following oxoacids of sulphur contains "S" in two different oxidation states?
\( \mathrm{H_2S_2O_3} \)
\( \mathrm{H_2S_2O_6} \)
\( \mathrm{H_2S_2O_7} \)
\( \mathrm{H_2S_2O_8} \)