Pure water would have a $BOD$ value of

The condition that indicates a polluted environment is

The incorrect statement from the following is:

What does the troposphere contain?

Which one of the following gases converts haemoglobin into carboxyhaemoglobin?

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.