Which one of the following is the true covalent oxide of iodine?

Manganese dioxide on heating with concentrated hydrochloric acid gives a greenish yellow gas,$X$. With excess of ammonia,$X$ gives $Y$ and $Z$. $Y$ and $Z$ respectively are:

Which is used for sterilization of water in the water supply system of cities?

Which of the following halogens forms the maximum number of oxoacids?

Which red-brown gas is evolved when $KCl$ is heated with solid $K_2Cr_2O_7$ and concentrated $H_2SO_4$?

Identify the correct pairs in which the chemical substance given is correctly matched with its use.

Chemical substance	Use
$A. Cl_2$	Preparation of phosgene
$B. I_2O_5$	Estimation of $CO$
$C. O_3$	Disinfectant