When a substance A reacts with water it produces a combustible gas B and a solution of substance C in water. When another substance D reacts with this solution of C, it also produces the same gas B on warming but D can produce gas B on reaction with dilute sulphuric acid at room temperature. A imparts a deep golden yellow colour to a smokeless flame of Bunsen burner. 

The substance A, B, C and D, respectively are:
1.  K, $H_2$, NaOH, Zn
2.  Na, NaOH, $H_2$ , Zn
3.  Li, $H_2$, LiOH, Zn
4.  None of the above

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:

1. ${\mathrm{CaCO}}_3$

2. ${\mathrm{Na}}_2{\mathrm{CO}}_3$

3. ${\mathrm{K}}_2{\mathrm{CO}}_3$

4. $\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2$