Which of the following yields $H_2O_2$ upon oxidation?

The concentration of commercial $H_2O_2$ is $3.125 \ M$,then its grade is $..........$ volume.

Decomposition of $H_2O_2$ is prevented by

The equation $H_2S + H_2O_2 \to S + 2H_2O$ represents:

$H_2O_2$ is used as:

$1 \ mL$ of "$x$ volume" $H_2O_2$ solution on heating gives $20 \ mL$ of oxygen gas at $STP$. The $(w/v) \%$ corresponding to "$x$ volume" of $H_2O_2$ is