Explanation:

A-P; B-Q; C-S; D-R

Currently, most hydrogen is produced from fossil fuels, specifically natural gas. Electricity—from the grid or from renewable sources such as wind, solar, geothermal, or biomass—is also currently used to produce hydrogen. In the longer term, solar energy and biomass can be used more directly to generate hydrogen.

Air is mostly gas
It's a mixture of different gases. The air in Earth's atmosphere is made up of approximately 78 percent nitrogen and 21 percent oxygen. Air also has small amounts of lots of other gases, too, such as carbon dioxide, neon, and hydrogen.

Boron is a mineral that is found in food such as nuts and the environment. People take boron supplements as medicine. Boron is used for boron deficiency, menstrual cramps, and vaginal yeast infections.

Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues strontium and barium.