An inorganic compound $\left(\mathrm{A}\right)$ made of two most occurring elements in the earth’s crust and it is used in building construction when made to react with carbon forms a poisonous gas $\left(\mathrm{B}\right)$ which is diatomic molecule. Compounds $\left(\mathrm{A}\right)$ and $\left(\mathrm{B}\right)$ are