Assertion : Windowpanes of old building become thicker at the bottom than at the top .
Reason : Glass is an extremely viscous liquid .

Options

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

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion**: The assertion states that windowpanes of old buildings become thicker at the bottom than at the top. This is a common observation in older buildings, and we need to explore why this happens.

2. **Analyzing the Reason**: The reason given is that glass is an extremely viscous liquid. To evaluate this, we need to understand the nature of glass.

3. **Nature of Glass**: Glass is classified as an amorphous solid. This means it does not have a regular crystalline structure. It can also be described as a supercooled liquid, which implies that it has properties similar to liquids but behaves like a solid at room temperature.

4. **Viscosity of Glass**: Viscosity is a measure of a fluid's resistance to flow. High viscosity means that the fluid flows very slowly. In the case of glass, it has a very high viscosity, which means it does not flow easily.

5. **Flow of Glass**: Due to its high viscosity, glass flows downwards very slowly over time. This slow flow allows the glass at the bottom of the windowpane to accumulate more material than the glass at the top, leading to the observed thickness at the bottom.

6. **Conclusion**: Both the assertion and the reason are correct. The assertion is true because windowpanes do become thicker at the bottom, and the reason is also true because the high viscosity of glass explains why this thickening occurs.

7. **Final Evaluation**: Since both the assertion and reason are correct and the reason provides a valid explanation for the assertion, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.

**Final Answer**: Option A: Both assertion and reason are true, and the reason is the correct explanation of the assertion.

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