Which one of the following statements is not true ?

Options

A. pH of drinking water should be between 5.5-9.5
B. Concentration of DO below 6 ppm is good for the growth of fish
C. Clean water would have a BOD value of less than 5 ppm
D. Oxides of sulphur, nitrogen and carbon, are the most wider spread air pollutant

Answer

To solve the question of identifying which statement is not true, we will analyze each option provided in the question step by step.

### Step 1: Analyze Option 1
**Statement:** pH of drinking water should be between 5.5 to 9.5.

- **Explanation:** The pH scale ranges from 0 to 14. A pH of 7 is neutral, below 7 is acidic, and above 7 is basic. The acceptable range for drinking water is indeed between 5.5 and 9.5.
- **Conclusion:** This statement is **true**.

### Step 2: Analyze Option 2
**Statement:** The concentration of Dissolved Oxygen (DO) below 6 ppm is good for the growth of fish.

- **Explanation:** The ideal concentration of dissolved oxygen for fish is around 8 mg/L (which is equivalent to 8 ppm). Values below 6 ppm can lead to stress and diseases in fish. Concentrations below 2 mg/L are lethal.
- **Conclusion:** This statement is **not true**.

### Step 3: Analyze Option 3
**Statement:** Clean water would have a BOD value of less than 5 ppm.

- **Explanation:** BOD (Biochemical Oxygen Demand) is a measure of the amount of oxygen that microorganisms will consume while decomposing organic matter in water. Clean water typically has a BOD of less than 5 ppm, indicating low levels of pollution.
- **Conclusion:** This statement is **true**.

### Step 4: Analyze Option 4
**Statement:** Oxides of Sulphur, Nitrogen, and Carbon are the most widespread air pollutants.

- **Explanation:** These oxides are indeed significant contributors to air pollution, primarily from sources like motor vehicles and industrial processes. They have harmful effects on health and the environment.
- **Conclusion:** This statement is **true**.

### Final Conclusion
After analyzing all the statements, we find that the **second option** is the one that is **not true**.

### Summary of the Solution:
- **Option 1:** True
- **Option 2:** Not True (This is the answer)
- **Option 3:** True
- **Option 4:** True