The correct answer is D. All the above.
Hydrogen sulfide (H2S), carbon dioxide (CO2), and methane (CH4) are all gases that are commonly found in sewers. Hydrogen sulfide is a colorless, flammable gas with a rotten egg smell. It is produced by the decomposition of organic matter in the sewer. Carbon dioxide is a colorless, odorless gas that is produced by the respiration of bacteria in the sewer. Methane is a colorless, odorless, flammable gas that is produced by the decomposition of organic matter in the sewer.
All three of these gases can be dangerous to human health. Hydrogen sulfide can cause respiratory problems, headaches, and nausea. Carbon dioxide can cause asphyxiation. Methane can cause explosions.
It is important to be aware of the dangers of these gases and to take precautions to avoid exposure. If you are working in a sewer, it is important to wear a respirator and to have a way to ventilate the area. If you smell gas in a sewer, it is important to evacuate the area immediately and to call for help.