Given below are two statements:

Statement I: During Electrolytic refining, the pure metal is made to act as anode and its impure metallic form is used as cathode.

Statement II: During the Hall-Heroult electrolysis process, purified $\mathrm{Al}_{2} \mathrm{O}_{3}$ is mixed with $\mathrm{Na}_{3} \mathrm{AlF}_{6}$ to lower the melting point of the mixture.

In the light of the above statements, choose the most appropriate answer from the options given below:

Formulae for Nessler's reagent is :

Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R.

Assertion A: can be easily reduced using $\mathrm{Zn}-\mathrm{Hg} / \mathrm{HCl}$ to

Reason $\mathrm{R}: \mathrm{Zn}-\mathrm{Hg} / \mathrm{HCl}$ is used to reduce carbonyl group to $-\mathrm{CH}_{2}-$ group.

In the light of the above statements, choose the correct answer from the options given below:

The water quality of a pond was analyzed and its BOD was found to be 4. The pond has :