What is the main source of chlorofluorocarbon $(CFC)$ pollution?

The eutrophication of water body results in ...... .

Give a brief explanation about the international standards for drinking water.

Fill in the blanks:
$(1)$ $.......$ is made using fly ash and slag from the steel industry.
$(2)$ Biogas is used in the production of $.......$ and its byproduct is used as $.......$.

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R:$
Assertion $A:$ Polluted water may have a value of $BOD$ of the order of $17 \, ppm$.
Reason $R:$ $BOD$ is a measure of oxygen required to oxidise both the biodegradable and non-biodegradable organic material in water.
In the light of the above statements,choose the most appropriate answer from the options given below.

Which one of the following statements is not true?