BOD (Biochemical Oxygen Demand) is a measure of the amount of oxygen consumed by microorganisms in the breakdown of organic matter.
A BOD value of 0-5 indicates very clean water, as there is little organic matter to decompose.
A BOD of 6-15 indicates moderately polluted water.
A BOD higher than 15 suggests highly polluted water.
Since the BOD is 4, it indicates very clean water.
Thus, the correct answer is: (3) Very clean water.
Nature of compounds TeO₂ and TeH₂ is___________ and ______________respectively.
Consider the following sequence of reactions : 
Molar mass of the product formed (A) is ______ g mol\(^{-1}\).
The magnitude of heat exchanged by a system for the given cyclic process ABC (as shown in the figure) is (in SI units):

The following are the major sources of Water Pollution: