What is a defining feature of the entrance door of a Type A1 school bus?

A defining feature of the entrance door of a Type A1 school bus is that it is located behind the front wheels. This design helps to ensure the safety of passengers by creating a clear separation between the entry point of the bus and the front of the vehicle, where it might conflict with the activities of traffic or other vehicles. Additionally, the positioning behind the front wheels minimizes the risk of injury to students entering or exiting the bus, as it allows more visibility for both the driver and the students.

The other choices do not accurately represent this defining characteristic of the Type A1 school bus. The entrance door's location is crucial for safety regulations and operational efficiency as students load and unload. Other designs for doors may have different features depending on the bus type, but rear positioning relative to the front wheels is distinctly characteristic of Type A1 school buses.