Assertion: Magnetic field can not change the kinetic energy of moving charges.
Reason: Magnetic field can not change velocity vector.

Options

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

Answer

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

### Step 1: Understand the Assertion
The assertion states that "Magnetic field cannot change the kinetic energy of moving charges."

- **Explanation**: The kinetic energy (KE) of a charged particle is given by the formula:
\[ KE = \frac{1}{2}mv^2 \]
where \( m \) is the mass of the particle and \( v \) is its speed. The magnetic force acts perpendicular to the velocity of the charged particle. Since work done is defined as:
\[ W = F \cdot d \]
where \( F \) is the force and \( d \) is the displacement, if the force is perpendicular to the displacement, the work done is zero:
\[ W = Fd \cos(90^\circ) = 0 \]
Therefore, since no work is done, the kinetic energy of the moving charge remains constant.

### Step 2: Understand the Reason
The reason states that "Magnetic field cannot change velocity vector."

- **Explanation**: The velocity vector of a charged particle in a magnetic field does change. While the speed (magnitude of velocity) remains constant, the direction of the velocity vector changes due to the magnetic force acting on the charge. This results in circular motion if the velocity is perpendicular to the magnetic field. Hence, the statement that the magnetic field cannot change the velocity vector is incorrect because the direction of the velocity vector is indeed changing.

### Step 3: Conclusion
- The assertion is **true**: A magnetic field does not change the kinetic energy of moving charges.
- The reason is **false**: A magnetic field does change the direction of the velocity vector of moving charges.

### Final Answer
Since the assertion is true and the reason is false, the correct option is that the assertion is true but the reason is false.

---