Assertion `:-` While ascending up a mountain , one gets less oxygen resulting in hypoxia.
Reason `:-` Percentage of oxygen decreases in the air at high altitudes.

Options

A. If both Assertion & Reason are True & the Reasonn is a correct explanation of the Assertion .
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertion
C. If Assertion is True but the Reason is False.
D. If both Assertion & Reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that while ascending a mountain, one gets less oxygen, resulting in hypoxia. Hypoxia refers to a condition where there is a deficiency of oxygen in the tissues.

2. **Understanding the Reason**: The reason provided is that the percentage of oxygen decreases in the air at high altitudes. It is important to analyze whether this statement is true and how it relates to the assertion.

3. **Analyzing Air Pressure at High Altitudes**: At high altitudes, the air pressure decreases. This decrease in pressure means that there are fewer gas molecules, including oxygen, available in a given volume of air.

4. **Breathing Mechanism**: Under normal conditions, the atmospheric pressure is higher than the pressure inside the thoracic cavity (negative pressure). This pressure gradient allows air to flow into the lungs. However, at high altitudes, the reduced air pressure makes it more difficult to draw air into the lungs.

5. **Linking Assertion and Reason**: While the assertion is true (less oxygen leads to hypoxia), the reason provided is also true but does not fully explain the hypoxia condition. The hypoxia is primarily due to the decreased air pressure rather than a decrease in the percentage of oxygen.

6. **Conclusion**: Both the assertion and the reason are true, but the reason does not correctly explain the assertion. Therefore, the correct answer is that the assertion is true, and the reason is true but not the correct explanation.

### Final Answer:
The correct answer is Option B: Assertion is true, Reason is true, but Reason is not the correct explanation for Assertion.

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