Hint: Dissolved oxygen denotes the total amount of oxygen present in the water body. On the other hand, biochemical oxygen demand or BOD is a measure of oxygen required by microbes. This oxygen is used in the breakdown of organic matter present in water.
Complete answer: DO or dissolved oxygen is the total amount of oxygen present or dissolved in a given water sample or water body. Its measuring unit is ppm. The BOD or Biochemical oxygen demand is the measure of the oxygen required by the aerobic microorganisms present in the water body. The microorganisms use oxygen to degrade the available organic matter in the water body at a certain temperature. The natural water system has a BOD of 4 ppm. DO is the most essential unit of any water body as it reflects the health of the water body. It is significant as it provides support to the aquatic life within the water body. If DO levels go down in the water the aquatic life suffers and even begins to die. This will increase the organic decomposable matter in the water. Bacteria will get to dissolve more organic matter. Thus, the BOD will increase. Therefore, from the above discussion, we can conclude that the lower the DO of the water, the more will be BOD. Wastewaters have more BOD levels than DO as they have more levels of organic matter to be degraded.
So, the correct answer is option B.
Note: It is important to monitor the DO levels of the water body as it gives a measure for the BOD levels in the water. The measure of DO will determine the sustainability of the water body. The wastewater treatment plant uses these measures to restore healthy levels of DO in water.