A solid compound 'X' on heating gives $CO_2$ gas and a residue. The residue mixed with water forms 'Y'. On passing an excess of $CO_2$ through 'Y' in water, a clear solution, 'Z' is obtained. On boiling 'Z', compound 'X' is reformed. The compound 'X' is :-

Question

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