Question

what is a cathode - ray tube used to demonstrate? the presence of charged particles. the flow of water. the behavior of light. the movement of sound waves.

Explanation:

A cathode - ray tube is an evacuated glass tube. Electrons (charged particles) are emitted from a cathode and travel towards an anode. It is used to demonstrate the presence of charged particles. It has nothing to do with the flow of water, behavior of light, or movement of sound waves.

Answer:

The presence of charged particles.