$A$ compound is in its low oxidation state. Then it will be

For the given reaction,which of the following is not used to change the colour of the solution from orange to green?
$K_2Cr_2O_7 / H^{+} + y \to Cr^{+3} + \text{Others}$
$(\text{orange}) \quad \quad \quad (\text{green})$

Consider the following reaction:
$BrCH_2CH_2F + SbF_5 \xrightarrow{SO_2, -60^{\circ}C} CH_2-CH_2-Br^+ + SbF_6^-$
In this reaction,$SbF_5$ acts as:

When heated in air,brown copper powder turns black. This black powder would turn brown again when heated with:

$A$ filter paper soaked in salt $X$ turns brown when exposed to $HNO_{3}$ vapor. The salt $X$ is

$(A) \ HOCl + H_2O_2 \rightarrow H_3O^{+} + Cl^{-} + O_2$
$(B) \ I_2 + H_2O_2 + 2 OH^{-} \rightarrow 2 I^{-} + 2 H_2O + O_2$
Choose the correct option.