Assertion : All molecules in a gas have same speed.
Reason : Gas contains molecules of different size and shape.

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. **Understanding the Assertion**:
- The assertion states that "All molecules in a gas have the same speed."
- This statement is incorrect because, in reality, gas molecules are in constant motion and have a range of speeds due to their kinetic energy.

2. **Understanding the Reason**:
- The reason states that "Gas contains molecules of different size and shape."
- This statement is also incorrect. While gases can consist of different types of molecules, within a particular gas (like oxygen or nitrogen), the molecules are generally similar in size and shape.

3. **Conclusion**:
- Since both the assertion and the reason are incorrect, we conclude that neither statement is true.

### Final Answer:
Both the assertion and the reason are false.