Identify the ore of magnesium from the following.

Why does the solubility of alkaline earth metal carbonates and sulphates decrease down the group?

In which of the following exothermic reactions,the heat liberated per mole is the highest?

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

Heating compound $(A)$ produces a colorless gas and a residue $(B)$ which dissolves in water. Passing excess $CO_2$ gas through the aqueous solution of $(B)$ yields compound $(C)$,which is obtained in solid state. Gently heating compound $(C)$ gives back the white solid $(A)$. What is compound $(A)$?

The most stable carbonate is