A colourless solid (X) on heating evolved `CO_(2)` and also gave a white residue , soluble in water. Residue also gave `CO_(2)` when treated with silute acid. (X) is

A colourless solid substance (A) on heating evolved CO_2 and also gave a white residue, soluble in water. Residue also gave CO_2 when treated with dilute HCI.

A compound (A) on heating in Busen flame imparts brick red colouration. (A) on heating gives CO_(2) gas and a residue (B) . The residue (B) when treated with water gives (c ) . On passing an excess of CO_(2) through (C ) in water, a clear solution (D) is obtained. On boiling (D) , compound (A) is reformed. Residue (B) is