A particle in one-dimensional motion:
| 1. | with zero speed at an instant may have non-zero acceleration at that instant. | 
| 2. | with zero speed may have non-zero velocity. | 
| 3. | with constant speed, must have non-zero acceleration. | 
| 4. | with a positive value of acceleration must be speeding up. | 
The figure gives a speed-time graph of a particle in motion along the same direction. Three equal intervals of time are shown. In which interval is the average acceleration greatest in magnitude?
  
| 1. | Interval 2 | 2. | Interval 1 | 
| 3. | Interval 3 | 4. | Equal in all intervals |