Lichens are called air pollution indicators. Justify the statement.
 

Lichens can be used as air pollution indicators, especially of the concentration of sulfur dioxide in the atmosphere. Factories can cause air pollution Lichens are plants that grow in exposed places such as rocks or tree bark. They need to be very good at absorbing water and nutrients to grow there. Rainwater contains just enough nutrients to keep them alive. Air pollutants dissolved in rainwater, especially sulfur dioxide, can damage lichens, and prevent them from growing. This makes lichens natural indicators of air pollution. For example: bushy lichens need really clean air leafy lichens can survive a small amount of air pollution crusty lichens can survive in more polluted air. In places where no lichens are growing, it is often a sign that the air is heavily polluted with sulfur dioxide.