Which of the following fluorides has the lowest melting point?

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 of the following contains both calcium and magnesium?

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

Match List-$I$ with List-$II$.

List-$I$	List-$II$
$a$. $Ca(OCl)_2$	$i$. Antacid
$b$. $CaSO_4 \cdot \frac{1}{2} H_2O$	$ii$. Cement
$c$. $CaO$	$iii$. Bleach
$d$. $CaCO_3$	$iv$. Plaster of Paris

Choose the most appropriate answer from the options given below:

The incorrect order of solubility in water is