The BOD value of a water sample is a measure of the amount of oxygen consumed by microorganisms during the breakdown of organic matter in the sample. The higher the BOD value, the more polluted the water is likely to be.
In this case, the BOD values of the three water samples were recorded as follows:
Sample A: 20 mg/L
Sample B: 8 mg/L
Sample C: 400 mg/L
From these values, it is clear that sample C has the highest BOD value and is therefore the most polluted. Sample A has a relatively low BOD value, indicating that it is the cleanest of the three samples.
Assuming that sample A is the river water (relatively clean), we can assign the labels as follows:
Sample A: River water
Sample B: Untreated sewage water
Sample C: Secondary effluent discharged from a sewage treatment plant
This is because untreated sewage water is likely to have a higher BOD value than river water, while secondary effluent discharged from a sewage treatment plant may still contain significant amounts of organic matter and have a high BOD value, despite undergoing some level of treatment.