Because it is a relatively clean-burning fossil fuel,and it produces about half as much carbon dioxide as burning an equivalent amount of coal,there is increasing interest in _____________ ____________ as an energy source.

Carbon Dioxide

A colorless, odorless gas produced by burning carbon and organic compounds and by respiration, a key greenhouse gas contributing to global warming.

Coal

A combustible black or brownish-black sedimentary rock used primarily as a fuel in electricity generation.