$A$ solid compound $X$ on heating gives $CO_2$ gas and a residue. The residue mixed with water forms compound $Y$. On passing excess $CO_2$ gas through $Y$,a clear solution $Z$ is formed. On heating $Z$,compound $X$ is recovered. What is compound $X$?
$\mathop X\limits_{solid}$ $\xrightarrow{\text{heating}} CO_2 + \text{residue}$ $\xrightarrow{H_2O} Y$ $\xrightarrow{\text{excess } CO_2} \mathop Z\limits_{\text{solution}}$ $\xrightarrow{\text{heating}} X$

• A
  $Ca(HCO_3)_2$
• B
  $CaCO_3$
• C
  $Na_2CO_3$
• D
  $K_2CO_3$