Which of the following are the most suitable indicators of $S{O}_2$ pollution in the environment?

LICHENS: It is the symbiotic association (i.e., mutually useful association) of algae and fungi. The algal component of this association is called phycobiont (autotrophic) and fungal component is called mycobiont (heterotrophic). In this association, algae prepare food for fungi and fungi provides shelter and absorb mineral nutrients and water for its partner. Lichens are the indicator of air pollution, especially SO₂ pollution i.e. they do not grow in polluted areas. Phycobionts of lichen are sensitive to SO₂.