A compound 'X' on heating gives a colourless gas. The residue is dissolved in water to obtain 'Y'. Excess CO2 is passed through aqueous solution of 'Y' when 'Z' is formed. 'Z' on gentle heating gives back 'X'. The compound 'X' is:
a. NaHCO3
b. Na2CO3
c. Ca(HCO3)2
d. CaCO3