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.