Which among the following halogens does not form a polyhalide ion?

The gas evolved on heating $CaF_2$ and $SiO_2$ with concentrated $H_2SO_4$,on hydrolysis gives a white gelatinous precipitate. The precipitate is

Which of the following statements is correct?

$A$ solid $A$ which has photographic effect reacts with the solution of a sodium salt $B$ to give a pale yellow precipitate $C$. The sodium salt $B$ on heating with concentrated $H_2SO_4$ gives brown vapours. Identify $A$,$B$,and $C$.

Which pair of halogens does not undergo a disproportionation reaction with water?

Which of the following reagents does not oxidize $HCl$?