उत्तर : 1. "Air pollution"
व्याख्या :

Answer: A) Air pollution Explanation: 1. Lichens are plants that grow in exposed places such as rocks or tree bark.   2.  Lichens are widely used as environmental indicators or bio-indicators. They can be used as air pollution indicators, especially of the concentration of sulfur-di-oxide in the atmosphere.     3. They need to be very good at absorbing water and nutrients to grow there.