Which of the following salts becomes Plaster of Paris on being appropriately heated?

$CaCO_3$ $\xrightarrow[excess \ H_2O]{in \ CO_2} A$ $\xrightarrow{\Delta} B$
Identify $A$ and $B$.

$X + C + Cl_2 \xrightarrow{1000 \ K} Y + CO;$
$Y + 2H_2O \rightarrow Z + 2HCl$
Compound $Y$ is found in a polymeric chain structure and is an electron-deficient molecule. $Y$ must be:

$X$-ray tube windows are made using which of the following metals?

Chemical $A$ is used for softening temporary hard water. $A$ reacts with sodium carbonate to produce caustic soda. When $CO_2$ is passed through the solution of $A$,it turns milky. What is the chemical formula of $A$?

The hydroxide of alkaline earth metal,which has the lowest value of solubility product $(K_{sp})$ at normal temperature $(25^{\circ} C)$ is