The condition that indicates a polluted environment is

The concentration of dissolved oxygen $(DO)$ in cold water can go up to ......... $ppm$.

What are pesticides and herbicides? Explain giving examples.

Planting more and more trees helps to

Given below are two statements:
Statement $I$: Non-biodegradable wastes are generated by the thermal power plants.
Statement $II$: Bio-degradable detergents lead to eutrophication.
In the light of the above statements,choose the most appropriate answer from the options given below:

Given below are two statements :
Statement $I$ : If $BOD$ is $4\,ppm$ and dissolved oxygen is $8\,ppm$,then it is a good quality water.
Statement $II$ : If the concentration of zinc and nitrate salts are $5\,ppm$ each,then it can be a good quality water.
In the light of the above statements,choose the most appropriate answer from the options given below :