The gas evolved in sewers is A. Carbondioxide B. Hydrogen sulphide C. Methane D. All of these

The correct answer is D. All of these.

Carbon dioxide, hydrogen sulfide, and methane are all gases that can be found in sewers. Carbon dioxide is produced by the respiration of bacteria and other organisms in the sewer. Hydrogen sulfide is produced by the decomposition of organic matter. Methane is produced by the anaerobic digestion of organic matter.

These gases can be harmful to human health if they are inhaled in high concentrations. Carbon dioxide can cause asphyxiation. Hydrogen sulfide can cause respiratory problems, headaches, and nausea. Methane can cause explosions.

It is important to ventilate sewers properly to prevent the buildup of these gases.