_ are very good bioindicators of air quality as they are very sensitive to pollution.

Explanation

Answer is "Lichens"Explanation: They are sensitive to air pollution particularly to SO2 and start dieing immediately if air becomes polluted.Lichens are very good bioindicators of air quality as they are very sensitive to pollution. Lichens are organisms that are formed by a symbiotic relationship between a fungus and an alga. They are found in a variety of environments, but they are particularly sensitive to air pollution.