Which gases are formed by the oxidation of carbon with concentrated $H_2SO_4$?

When mercury $(II)$ chloride is treated with excess of $SnCl_2$,the products obtained are

Assign $A, B, C, D$ from the given type of reaction.
$Hg(NO_3)_2 + NH_3(soln.) \longrightarrow HgO \cdot HgNH_2NO_3 \downarrow $

$500$ mL of $0.2$ $M$ $MnO_4^-$ solution in basic medium when mixed with $500$ mL of $1.5$ $M$ $KI$ solution, oxidises iodide ions to liberate molecular iodine. This liberated iodine is then titrated with a standard $x$ $M$ thiosulphate solution in presence of starch. If $300$ mL of thiosulphate was consumed, then the value of $x$ is . . . . . . .

Calculate the millimoles of $SeO_3^{2-}$ in solution on the basis of following data: $70 \ mL$ of $\frac{M}{60}$ solution of $KBrO_3$ was added to $SeO_3^{2-}$ solution. The bromine evolved was removed by boiling and excess of $KBrO_3$ was back titrated with $12.5 \ mL$ of $\frac{M}{25}$ solution of $NaAsO_2$. The reactions are given below.
$I. \ SeO_3^{2-} + BrO_3^{-} + H^{+} \to SeO_4^{2-} + Br_2 + H_2O$
$II. \ BrO_3^{-} + AsO_2^{-} + H_2O \to Br^{-} + AsO_4^{3-} + H^{+}$

In basic medium,$CrO_{4}^{2-}$ oxidises $S_{2}O_{3}^{2-}$ to form $SO_{4}^{2-}$ and itself changes into $Cr(OH)_{4}^{-}$. The volume of $0.154 \ M \ CrO_{4}^{2-}$ required to react with $40 \ mL$ of $0.25 \ M \ S_{2}O_{3}^{2-}$ is ........... $mL$ (Rounded-off to the nearest integer).