All of the following substances react with water. Which of the following pairs will produce the same gaseous product upon reaction with water?

$A$ solid compound $X$ on heating gives $CO_2$ gas and a residue. The residue mixed with water forms compound $Y$. On passing excess $CO_2$ gas through $Y$,a clear solution $Z$ is formed. On heating $Z$,compound $X$ is recovered. What is compound $X$?
$\mathop X\limits_{solid}$ $\xrightarrow{\text{heating}} CO_2 + \text{residue}$ $\xrightarrow{H_2O} Y$ $\xrightarrow{\text{excess } CO_2} \mathop Z\limits_{\text{solution}}$ $\xrightarrow{\text{heating}} X$

Among the metals $Be$,$Mg$,$Ca$,and $Sr$ of group $IIA$,which one forms the least ionic chloride in the periodic table?

The solubility in water of sulphates down the $Group \ 2$ ($Be$ group) is $Be > Mg > Ca > Sr > Ba$. This is due to

Plaster of Paris is

Give the biological importance of calcium and magnesium.