A system offers four service classes A, B, C and D, which differ in price tag and have a distinctive public method doA(), doB(), doC(), and doD(), respectively. These classes do share an identical public feature doCommon() method. The price attribute is private and is declared only once somewhere to be shared by all service classes. To use the system, a selected service is first paid for and the corresponding object is created to run the distinctive method and then the doCommon() method. Show the UML class diagram for the system. Extra interfaces, classes or methods can be added if needed.