Assertion: The angle between vectors `vecAxxvecB` and `vecBxxvecA` is `pi` radian.
Reason: `vecBxxvecA=-(vecAxxvecB)`

Options

A. torque and work
B. angular momentum and work
C. energy and Young's modulus
D. light year and wevelength

Answer

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
The assertion states that the angle between the vectors \( \vec{A} \times \vec{B} \) and \( \vec{B} \times \vec{A} \) is \( \pi \) radians (or 180 degrees).

2. **Understanding the Cross Product**:
The cross product of two vectors \( \vec{A} \) and \( \vec{B} \) is defined as:
\[ \vec{A} \times \vec{B} = |\vec{A}| |\vec{B}| \sin(\theta) \hat{n} \]
where \( \theta \) is the angle between \( \vec{A} \) and \( \vec{B} \), and \( \hat{n} \) is a unit vector perpendicular to the plane containing \( \vec{A} \) and \( \vec{B} \).

3. **Direction of Cross Products**:
By the right-hand rule, the direction of \( \vec{A} \times \vec{B} \) is determined by curling the fingers of your right hand from \( \vec{A} \) to \( \vec{B} \), with your thumb pointing in the direction of \( \vec{A} \times \vec{B} \). Conversely, \( \vec{B} \times \vec{A} \) will point in the opposite direction.

4. **Relationship Between the Two Cross Products**:
It is known that:
\[ \vec{B} \times \vec{A} = -(\vec{A} \times \vec{B}) \]
This means that the vector \( \vec{B} \times \vec{A} \) is equal in magnitude but opposite in direction to \( \vec{A} \times \vec{B} \).

5. **Calculating the Angle**:
Since \( \vec{B} \times \vec{A} \) is in the opposite direction to \( \vec{A} \times \vec{B} \), the angle between them is:
\[ \text{Angle} = 180^\circ = \pi \text{ radians} \]

6. **Conclusion on the Assertion**:
Therefore, the assertion that the angle between \( \vec{A} \times \vec{B} \) and \( \vec{B} \times \vec{A} \) is \( \pi \) radians is true.

7. **Understanding the Reason**:
The reason states that \( \vec{B} \times \vec{A} = -(\vec{A} \times \vec{B}) \). This is a fundamental property of the cross product and confirms that the direction of \( \vec{B} \times \vec{A} \) is indeed opposite to that of \( \vec{A} \times \vec{B} \).

8. **Conclusion on the Reason**:
Since the reason correctly explains why the assertion is true, we can conclude that both the assertion and the reason are true, and the reason is the correct explanation for the assertion.

### Final Answer:
Both the assertion and the reason are true, and the reason is the correct explanation for the assertion.