(1) Coal combustion will produce harmful gases such as sulfur dioxide, which will cause environmental pollution. Wind energy, hydrogen energy and solar energy are 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: carbon dioxide; 2H2O？ Catalyst? . ? 2H2↑+O2↑。

(3) The main component of natural gas is methane;

So the answer is CH4.

(4) Solution: mass fraction of solute = mass of solute solution × 100%.

The obtain solute mass = solution mas × solute mass fraction.

= 100×98%

=98 (gram)

Let the mass of distilled water to be added be X, and substitute the dilution solution data into the relational expression: solute mass fraction = solute mass × 100%.

Get 98 100+x× 100%=28%.

The solution is x=250 (g)