Biochemical oxygen demond (BOD) in a river water.

Options

A. Remain unchanged when algal bloom occurs
B. Gives a measure of salmonella in the river
C. Increases when sewage gets mixed with river water
D. All of the above

Answer

**Step-by-Step Solution:**

1. **Understanding Biochemical Oxygen Demand (BOD):**
- BOD is a measure of the amount of oxygen that microorganisms will consume while decomposing organic matter in water. It indicates the level of organic pollution in the water.

2. **Analyzing the Options:**
- **Option 1:** "BOD remains unchanged when algal bloom occurs."
- Algal blooms can lead to increased organic matter when they decompose, which would increase BOD. Thus, this statement is incorrect.

- **Option 2:** "Gives a measure of Salmonella in the river water."
- BOD is related to organic matter decomposition and does not directly measure Salmonella or any specific pathogens. Therefore, this statement is also incorrect.

- **Option 3:** "Increases when sewage gets mixed with river water."
- Sewage contains a significant amount of organic matter. When sewage mixes with river water, the organic matter increases, leading to a higher demand for oxygen by microorganisms to decompose this organic matter. Thus, this statement is correct.

3. **Conclusion:**
- The correct answer is that BOD increases when sewage gets mixed with river water. Therefore, the correct option is **Option 3**.

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