Mine Gases - No. 11 Questions and answers taken from PowerPoint Presentations found on this site. Progress Indicator: Question 1 of 25 1.  What is carbon dioxide (CO2)? Choices: It is a colorless gas with a foul odor which is combustible. It is a gas formed by the chemical combination of sulfur and oxygen. Carbon dioxide is a colorless, odorless, combustible gas. Carbon dioxide is a colorless and odorless gas formed by the chemical combination of carbon and oxygen.