渥太华这个城市正在创建一个有关交通工具的“数据库”,来为它的议会(委员会)做预算提供较好的参考。这个城市有许多不同种类的机动车辆：客车，货车，卡车，紧急车辆(救护，消防等)，在对这个城市了解之后，要求你设计一个有下图要求的层次的系统。-The city of Ottawa is creating a “database” of its vehicles fleet in order to provide better

support to the council in making budget decisions. The city has many vehicles of

different types: passenger cars, vans, trucks, and emergency vehicles.

After consultation with the city, you designed a system with the following hierarchy.