The correct answer is $\boxed{\text{B. 225 ppm}}$.
Biological oxygen demand (BOD) is a measure of the amount of oxygen that is used by microorganisms to break down organic matter in water. It is a useful indicator of the quality of water, as high BOD levels can indicate that the water is polluted with organic matter.
The BOD of a sample of sewage can be determined by incubating a known volume of the sample at a constant temperature for a specified period of time, and then measuring the amount of oxygen that is consumed. The BOD is expressed in milligrams of oxygen per liter (mg/L).
In the question, a 2% solution of sewage is incubated for 5 days at 20°C. The depletion of oxygen was found to be 5
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