$A$ compound $x$ on heating gives a colourless gas. The residue is dissolved in water to obtain $y$. Excess $CO_2$ is bubbled through the aqueous solution of $y$ and $z$ is formed. $z$ on gentle heating gives back $x$. The compound $x$ is:

• A
  $CaCO_3$
• B
  $Ca(HCO_3)_2$
• C
  $NaHCO_3$
• D
  $NaOH$