Water samples with BOD values of 4 ppm and 18 ppm, respectively are:

Options

A. Clean and clean
B. Highly polluted and clean
C. Highly polluted and highly polluted
D. Clean and highly polluted

Explanation

To solve the question regarding the BOD (Biological Oxygen Demand) values of two water samples, we will analyze the given BOD values and determine the condition of each water sample.

### Step-by-Step Solution:

1. **Understanding BOD**:
- BOD (Biological Oxygen Demand) measures the amount of oxygen that microorganisms will consume while decomposing organic matter in water. It is an important indicator of water quality.

2. **BOD Value Interpretation**:
- A BOD value of less than 5 ppm indicates clean water.
- A BOD value greater than 17 ppm indicates highly polluted water.

3. **Analyzing the First Water Sample**:
- The first water sample has a BOD value of 4 ppm.
- Since 4 ppm is less than 5 ppm, we classify this water sample as **clean**.

4. **Analyzing the Second Water Sample**:
- The second water sample has a BOD value of 18 ppm.
- Since 18 ppm is greater than 17 ppm, we classify this water sample as **highly polluted**.

5. **Conclusion**:
- The first water sample is clean (4 ppm).
- The second water sample is highly polluted (18 ppm).
- Therefore, the correct classification of the two water samples is: **clean and highly polluted**.

### Final Answer:
The water samples with BOD values of 4 ppm and 18 ppm, respectively, are **clean and highly polluted**.