The work done by kinetic friction on a body :

Options

A. is always negative
B. is always zero
C. may be negative or positive
D. is always positive

Explanation

To solve the question regarding the work done by kinetic friction on a body, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding Kinetic Friction**:
Kinetic friction is the force that opposes the motion of two surfaces sliding past each other. It acts in the direction opposite to the displacement of the object.

2. **Analyzing the Situation**:
Consider a block being pulled on a surface with a force \( F \). If there is another block on top of it, the interaction between the two blocks will also involve friction.

3. **Identifying Forces**:
- For the block being pulled (let's call it Block 1), the kinetic friction force acts in the opposite direction to the displacement.
- For the block on top (Block 2), if it moves along with Block 1, the kinetic friction force acts in the same direction as the displacement.

4. **Calculating Work Done**:
- The work done by a force is given by the formula:
\[ W = F \cdot d \cdot \cos(\theta) \]
where \( F \) is the force, \( d \) is the displacement, and \( \theta \) is the angle between the force and the displacement direction.
- For Block 1, since the friction force is opposite to the displacement, \( \theta = 180^\circ \) and \( \cos(180^\circ) = -1 \). Thus, the work done by kinetic friction on Block 1 is negative.
- For Block 2, if the friction force is in the same direction as the displacement, \( \theta = 0^\circ \) and \( \cos(0^\circ) = 1 \). Thus, the work done by kinetic friction on Block 2 is positive.

5. **Conclusion**:
The work done by kinetic friction can be negative (when it opposes the motion) or positive (when it aids the motion). There can also be cases where the work done is zero (for instance, if there is no displacement).

### Final Answer:
The work done by kinetic friction on a body may be negative or positive.