Cathode rays deflect in the presence of magnetic field.

Answer

### Step-by-Step Solution:

1. **Understanding Cathode Rays**:
- Cathode rays are streams of electrons emitted from the cathode (negative electrode) in a vacuum tube.

2. **Nature of Electrons**:
- Electrons are negatively charged particles. This charge is crucial for understanding their behavior in a magnetic field.

3. **Effect of Magnetic Fields on Charged Particles**:
- When charged particles, such as electrons, move through a magnetic field, they experience a force. This force is described by the Lorentz force law, which states that the force (F) acting on a charged particle is proportional to the charge (q), the velocity (v) of the particle, and the magnetic field (B).
- The formula is:
\[ F = q(v \times B) \]
- The direction of the force is perpendicular to both the velocity of the particle and the magnetic field.

4. **Deflection of Cathode Rays**:
- Since cathode rays consist of electrons, when they pass through a magnetic field, they will be deflected due to the force acting on them. The direction of deflection will depend on the orientation of the magnetic field and the direction of the electron's motion.

5. **Conclusion**:
- Therefore, the statement "Cathode rays deflect in the presence of a magnetic field" is true because the electrons in the cathode rays are charged particles that respond to magnetic fields.