Answer these questions:

a) When should abstract classes be used ? Give a general answer and an example.

b) Explain when an abstract class cannot be used in a Java program and an interface is the only choice.

c) The Section class used in a text editor application includes a text attribute, contains a collection of objects of class Section or its subclasses, and has a method called display that displays the text of this section object and of all other sections referenced in its collection. What design pattern does class Section implement ? Explain.