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.
The heat generated in 1 minute between points A and B in the given circuit, when a battery of 9 V with internal resistance of 1 \(\Omega\) is connected across these points is ______ J. 
The following are the major sources of Water Pollution: