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

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 $A$?

Sorel's cement is

The compound $A$ on heating gives a colourless gas and a residue that is dissolved in water to obtain $B$. Excess of $CO_2$ is bubbled through aqueous solution of $B$,$C$ is formed which is recovered in the solid form. Solid $C$ on gentle heating gives back $A$. The compound $A$ is:

Choose the incorrect statement.

The correct sequence of thermal stability of the following carbonates is: