A solid compound 'X' on heating gives CO₂ gas and a residue. The residue mixed with water forms 'Y'. On passing an excess of CO₂ through 'Y' in water, a clear solution 'Z' is obtained. On boiling 'Z', compound 'X' is reformed. The compound 'X' is :
a.Na2CO₂
b.Ca(HCO₃)₂
c.CaCO₃
d.K₂CO₃
