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:

• A
  $CaCO_3$
• B
  $Na_2CO_3$
• C
  $K_2CO_3$
• D
  $CaSO_4 \cdot 2H_2O$