A vehicle is directly behind the rear bumper in the flat-mirror blind spot. What does this illustrate about school bus mirror use?

1. Flat mirrors have a blind spot directly behind the rear bumper, so drivers must use full mirror checks and avoid assuming the rear is clear
2. Flat mirrors eliminate all blind spots if adjusted once a year
3. The overhead inside mirror always shows the full rear danger zone
4. Rear blind spots matter only when the bus is empty