One Sentence summary of the purpose of the UML Model:

1. Use Cases - "How will our system interact with the outside world?"
2. Class Diagram - "What objects do we need? How will they be related?"
3. Collaboration Diagram - "How will the object interact?"
4. Sequence Diagram - "How will the objects interact?"
5. State Diagram - "What stats should our objects be in?"
6. Package Diagram -"Show are we going to modularise our development?"
7. Component Diagram -"How will our software components be related?"
8. Deployment Diagram -"How will the software be deployed?"

The passenger traveling at the fare being validated that is required to travel with another passenger.

In an interface class, all methods are abstract - there is no implementation. 

In an abstract class some methods can be concrete.  In an interface class, no accessibility modifiers are allowed. 

An abstract class may have accessibility modifiers. 
 

Yes.  .NET does support multiple interfaces. 

They all must be public, and are therefore public by default. 

Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces do not provide implementation. They are implemented by classes, and defined as separate entities from classes. 

A class that cannot be instantiated.  An abstract class is a class that must be inherited and have the methods overridden.  An abstract class is essentially a blueprint for a class  with or without implementation. 

Yes.  Just leave the class public and make the method sealed. 

Yes.  The keyword “sealed” will prevent the class from being inherited. 

The data value may not be changed.  Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory. 

System.String is immutable.  System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.