What is the primary physical cause of passenger discomfort known as 'jerk' (Ruck) during bus acceleration or braking?

1. The rapid rate of change of acceleration over time (da/dt), which prevents passengers from adjusting their balance muscularly
2. A constant high velocity maintained along straight highway sections
3. The steady, linear deceleration produced by an aerodynamically shaped bus body
4. The continuous baseline rolling resistance generated by dual rear tires