Assertion : Jet aeroplane flying at high altitude need pressurization of the cabin.
Reason : Oxygen is not present at higher altitude.

Options

A. If both assertion and reason are true and the reason is the correct explanation of the assertion.
B. If both assertion and reason are true but reason is not the correct explanation of the assertion.
C. If assertion is true but reason is false.
D. If the assertion and reason both are false.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understand the Assertion**:
- The assertion states that "jet aeroplanes flying at high altitude need pressurization of the cabin."
- This is true because as an airplane ascends to higher altitudes, the atmospheric pressure decreases significantly. To ensure the safety and comfort of passengers, the cabin must be pressurized to maintain a suitable environment.

2. **Understand the Reason**:
- The reason given is "Oxygen is not present at higher altitude."
- This statement is misleading. While it is true that the density of oxygen decreases at higher altitudes, oxygen is still present in the atmosphere. The issue is not the absence of oxygen but rather its lower availability due to reduced pressure.

3. **Evaluate the Relationship**:
- The assertion is true, but the reason is false. The need for cabin pressurization is due to the low pressure and low density of oxygen at high altitudes, not because oxygen is completely absent.

4. **Conclusion**:
- Since the assertion is true and the reason is false, the correct answer to the question is that the assertion is true, but the reason is not a valid explanation for the assertion.

### Final Answer:
The assertion is true, and the reason is false. Therefore, the correct option is option number 3.

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