Consider an electron, a neutron and a proton initially at rest and placed along a straight line such that the neutron is exactly at the center of the line joining the electron and proton. At $t=0$, the particles are released but are constrained to move along the same straight line. Which of these will collide first?
- The particles will never collide.
- All will collide together.
- Proton and neutron.
- Electron and neutron.