Oxalic acid when treated with potassium permanganate in the presence of an acid,produces

In the reaction $SnCl_2(excess) + HgCl_2 \rightarrow A + SnCl_4$,'$A$' is

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$FeSO_4 + Na_2S \longrightarrow FeS \downarrow$

$HNO_2$ acts as

In acidic medium, one mole each of $MnO_4^-$ and $Cr_2O_7^{2-}$ is reduced by $x$ and $y$ moles of ferrous ions $(Fe^{2+})$ respectively. The sum of $x$ and $y$ is:

Which of the following reactions does not involve either oxidation or reduction?
$I. \ 2Cs + 2H_2O \to 2CsOH + H_2$
$II. \ 2CuI_2 \to 2CuI + I_2$
$III. \ NH_4Br + KOH \to KBr + NH_3 + H_2O$
$IV. \ 4KCN + Fe(CN)_2 \to K_4[Fe(CN)_6]$