Which of the following statements is not true?

Options

A. Ozone layer occurs naturally in the stratosphere.
B. Chlorofluorocarbons are the major ozone depleting substances which produce active chlorine radicals in the presence of UV radiations.
C. Thinning of ozone layer allows more UV radiations to pass through it and cause harmful effects on humans, plants and animals.
D. The region of the stratosphere that contains relatively high concentration of ozone is called troposphere.

Explanation

To determine which statement is not true, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Evaluate Option A**:
- Statement: "Ozone layer occurs naturally in the stratosphere."
- Analysis: The ozone layer is indeed found in the stratosphere, where it plays a crucial role in absorbing harmful UV radiation from the sun.
- Conclusion: This statement is **true**.

2. **Evaluate Option B**:
- Statement: "Chlorofluorocarbons (CFCs) are major ozone-depleting substances which produce active chlorine radicals in the presence of UV radiations."
- Analysis: CFCs are known to release chlorine atoms when they are broken down by UV radiation, which leads to the depletion of the ozone layer.
- Conclusion: This statement is **true**.

3. **Evaluate Option C**:
- Statement: "Thinning of the ozone layer allows more UV radiations to pass through it and cause harmful effects on humans, plants, and animals."
- Analysis: The thinning of the ozone layer does indeed allow more UV radiation to reach the Earth's surface, which can lead to harmful effects such as skin cancer and damage to ecosystems.
- Conclusion: This statement is **true**.

4. **Evaluate Option D**:
- Statement: "The region of stratosphere that contains relatively high concentration of ozone is called troposphere."
- Analysis: This statement is incorrect. The region of the stratosphere with a high concentration of ozone is referred to as the "ozonosphere," not the troposphere. The troposphere is the layer of the atmosphere closest to the Earth's surface, where weather occurs.
- Conclusion: This statement is **not true**.

### Final Answer:
The statement that is not true is **Option D**.

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