What is the product formed when concentrated $HNO_3$ reacts with iodine?

Among the following acids,the weakest one is

$A$ 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 $C$.

Anhydrous ferric chloride is prepared by

Euchlorine is a mixture of .........

Which of the following elements has a half-life in milliseconds?