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) Na2CO3 (b) K2CO3
(c) Ca(HCO3 )2 (d) CaCO3