Clean water organisms disappears when the concentration of

Options

A. BOD is less
B. DO is least
C. DO is more
D. COD is more

Answer

### Step-by-Step Solution:

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

2. **Understanding DO (Dissolved Oxygen)**:
- DO is the amount of oxygen that is present in water and is essential for the survival of aquatic organisms like fish and other aquatic life.

3. **Analyzing the Options**:
- The question provides four options regarding the conditions under which clean water organisms disappear:
1. BOD is less
2. DO is less
3. DO is more
4. COD (Chemical Oxygen Demand) is more

4. **Evaluating Each Option**:
- **Option 1 (BOD is less)**: If BOD is low, it means there is less organic matter to decompose, leading to higher DO levels. This condition is beneficial for aquatic organisms, so this option is incorrect.
- **Option 2 (DO is less)**: If DO is low, aquatic organisms will not receive enough oxygen, leading to their mortality. This condition is detrimental to clean water organisms, making this option correct.
- **Option 3 (DO is more)**: Higher DO levels are favorable for aquatic life, so this option is incorrect.
- **Option 4 (COD is more)**: COD measures the total oxygen demand of all substances in water, including organic and inorganic matter. While high COD can indicate pollution, it does not directly relate to the disappearance of clean water organisms as effectively as low DO does. Therefore, this option is also incorrect.

5. **Conclusion**:
- The correct answer is that clean water organisms disappear when the concentration of DO is less.

### Final Answer:
Clean water organisms disappear when the concentration of **DO (Dissolved Oxygen) is less**.