Which of the following is the correct decreasing order of thermal stability of alkaline earth metal carbonates?

$A$ compound $X$ on heating gives a colourless gas. The residue is dissolved in water to obtain $Y$. Excess $CO_2$ is bubbled through the aqueous solution of $Y$ and $Z$ is formed. $Z$ on gentle heating gives back $X$. The compound $X$ is:

The thermal stability of alkaline earth metal carbonates $MgCO_{3}$,$CaCO_{3}$,$SrCO_{3}$,and $BaCO_{3}$ follows the order:

When water is added to compound $(A)$ of calcium,a solution of compound $(B)$ is formed. When carbon dioxide is passed into the solution,it turns milky due to the formation of compound $(C)$. If excess of carbon dioxide is passed into the solution,the milkiness disappears due to the formation of compound $(D)$. Identify the compounds $A$,$B$,$C$,and $D$. Explain why the milkiness disappears in the last step.

The thermal stability of $BaCO_3$,$CaCO_3$,$SrCO_3$,and $MgCO_3$ decreases in the order:

The metal which can be extracted from all three ores: dolomite,magnesite,and carnallite is: