Answer: 2. CaO (s) + CO2 (g) → CaCO3 (s) and
4. PbCO3 (s) → PbO (s) + CO2 (g)
In (2), limestone is decomposed under heating to give rise to quicklime and CO2, while (4), is the thermal decomposition of lead to produce PbO and CO2.
Explanation: In the first reaction, methane (CH4 (g)) undergoes a combustion reaction to produce CO2 and H2O.
The third reaction is a combination reaction.