What is B.O.D.:
1. The amount of $O_2$ utilized by organisms in the water
2. The amount of $O_2$ utilized by microorganisms for decomposition
3. The total amount of $O_2$ present in water
4. All of the above

Escherichia coli is used as an indicator organism to determine the pollution of water with:

1. Heavy metals

2. Faecal matter

3. Industrial effluents

4. Pollen of aquatic plants

A major component of gobar gas is:

1. Methane

2. Ethane

3. Butane

4. Ammonia 

Common indicator organism of water pollution is:

1. Eichhornia crassipes

2. Escherichia coli

3. Entamoeba histolytica

4. Lemna pancicostata