Thermal decomposition of which of the salt listed below yields a basic and an acidic oxide simultaneously?

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 $(A)$?

Which halide of alkaline earth metals is covalent in nature and can be soluble in organic solvent, such as ethanol?

Give a detailed explanation of the oxoacid salts of alkaline earth metal elements.

Describe the importance of the following:
$(i)$ limestone
$(ii)$ cement
$(iii)$ plaster of Paris.

Plaster of Paris is