Bioindicator

Q. Manu lived in a city with heavy traffic, factories, noisy airports etc. To take a break from the daily life, he left for his grandma's village.There, a grey powdery structure in rocks and bark of trees caught his attention. He has never seen such a thing in the busy streets of his city.
What could these structures be?

1. Algae
2. Lichens
3. Fungus
4. Mushroom

Q. ___ indicate(s) that a waterbody is polluted.

1. Presence of lichens
2. Absence of lichens
3. Absence of snails
4. Algal boom