Answer:
4, A proton in a particle accelerator moves faster and faster.
Step-by-step explanation:
A particle accelerator according to Wikipedia.com is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for basic research in particle physics.
As the proton moves around, it collides with other proton and the wall of the accelerator thereby undergoing net interaction.