Which of the following organisms is the main indicator of sulphur dioxide and nitrogen pollution?

Lichens are the main biological indicator of sulphur dioxide and nitrogen pollution, dying off where the air is foul.

Blue-green algae signal nutrient enrichment in water, not sulphur and nitrogen air pollution.

E. coli indicates faecal contamination of water, unrelated to these air pollutants.

With a clear indicator present, the choice denying any answer does not apply.

Lichens absorb pollutants directly from air as they lack a protective cuticle.

Their presence or absence maps long-term air quality across an area.