A : Average velocity of gas molecules is zero,
R : Due to random motion of gas molecules, velocities of different molecules cancer each other.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then Mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4).

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided.

### Step 1: Understand the Assertion (A)
The assertion states that the average velocity of gas molecules is zero.

- **Explanation**: Average velocity is defined as the total displacement of the gas molecules divided by the total time taken. In a gas, molecules move in all directions randomly. For every molecule moving in one direction, there is likely another molecule moving in the opposite direction. Therefore, when we calculate the average velocity, the displacements in opposite directions cancel each other out, resulting in an average velocity of zero.

### Step 2: Understand the Reason (R)
The reason states that due to the random motion of gas molecules, the velocities of different molecules cancel each other.

- **Explanation**: This statement is true because the random motion of gas molecules means that they are moving in all directions. For every molecule that moves in one direction, there is another molecule moving in the opposite direction. This randomness leads to a situation where the velocities effectively cancel each other out when we consider the average, leading to an average velocity of zero.

### Step 3: Conclusion
Both the assertion (A) and the reason (R) are correct. Furthermore, the reason (R) correctly explains the assertion (A).

### Final Answer
- Assertion (A) is correct.
- Reason (R) is correct and is the correct explanation of the assertion.