Assertion : Vapour density is the same thing as density.
Reason : Molecular weight is twice density.

Options

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

Explanation

To solve the given assertion and reason question, we will analyze both the assertion and the reason step by step.

### Step 1: Analyze the Assertion
- **Assertion**: "Vapour density is the same thing as density."
- **Explanation**: Vapour density refers specifically to the density of a gas compared to the density of hydrogen at the same temperature and pressure. It is calculated as the molar mass of the gas divided by 2 (when compared to hydrogen). In contrast, density is a general term that applies to solids, liquids, and gases and is defined as mass per unit volume (mass/volume). Therefore, vapour density and density are not the same.

### Conclusion for Step 1:
- The assertion is **false**.

### Step 2: Analyze the Reason
- **Reason**: "Molecular weight is twice density."
- **Explanation**: This statement suggests a direct relationship between molecular weight and density. However, density (D) is defined as mass (m) divided by volume (V), and molecular weight (M) is the mass of one mole of a substance. There is no general formula that states molecular weight is twice the density. In fact, density can vary based on temperature and pressure, while molecular weight is a constant for a given substance.

### Conclusion for Step 2:
- The reason is also **false**.

### Final Conclusion:
Both the assertion and the reason are incorrect. Therefore, the appropriate answer is that both the assertion and the reason are wrong.