a. burning coal

b. burning cooking gas

c. burning mosquito coil

d. room spray

Answer: a. burning coal

Explanation: Both the burning of coal and the burning of the mosquito coil will cause chemical pollution, but by comparison, the burning of coal will cause more pollution. Coal leads to the release of many pollutants from the air. Among these, CO is very dangerous, because it disrupts the respiratory system. People are often advised not to sleep with burning coals in a closed room.