Natural gas burns
cleaner than coal, yet it is considered a pollutant. Why?

Cleaner burning than other fossil fuels, the combustion of natural gas produces negligible amounts of sulfur, mercury, and particulates. Burning natural gas does produce nitrogen oxides (NOx), which are precursors to smog, but at lower levels than gasoline and diesel used for motor vehicles.

Explanation:

"a.) well–developed vascular system."

i think the answer would be d