Identify the wrong statement in the following:

How is biogas produced?

Summarize the primary information regarding water pollution.

$A$ large number of fish are suddenly found floating dead on a lake. There is no evidence of toxic dumping,but you find an abundance of phytoplankton. Suggest a reason for the fish kill.

Provide the correct statement for the following: (Use $T$ for True and $F$ for False statements.) $(1)$ Plants obtain elements like carbon,hydrogen,and oxygen from air and water. $(2)$ Plants obtain elements like $N, P, K, Ca$ from the soil. $(3)$ Compound fertilizers contain nitrogen as ammonium nitrate,phosphorus as $PCl_3$,and potassium as $KO_2$. $(4)$ The use of $NPK$ fertilizers increases the production of crops and vegetables in the soil.

Which of the following belongs to secondary air pollutants?