Only which kind of force affects the motion of a system?

1. A system's parts push on each other as well as on the outside world. 2. Only external forces affect the motion of a system, according to Newton's first law. 3. The internal forces actually cancel out. 4. You must define the boundaries of the system before you can determine which forces are external. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 4 'Dynamics: Force and Newton's Laws of Motion', sections 4.1-4.8_