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 $(A)$?

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