(1) Coal combustion will produce harmful gases such as sulfur dioxide, which will cause environmental pollution. Wind energy, hydrogen energy and solar energy are all new energy sources and will not cause environmental pollution. So choose b;

(2) Green plants can absorb water and carbon dioxide for "photosynthesis"; According to the data, the catalyst in the leaves will decompose water into hydrogen and oxygen, which will be stored in hydrogen-oxygen fuel cells for power generation. Catalyst? . ? 2H2↑+O2↑; So the answer is: CO2；; 2H2O？ Catalyst? . ? 2H2↑+O2↑;

(3) Methane combustion produces water and carbon dioxide, and the chemical equation is CH4+2O2? Light it? . ? CO2+2H2O； Based on the difference between heat production and carbon dioxide emission, the data of the two are compared and analyzed. The results show that methane has higher heat production and less carbon dioxide emission.

So the answer is: CH4+2O2? Light it? . ? CO2+2H2O； When burning two substances with the same mass, methane produces less carbon dioxide and gives off more heat.