Final answer:
To use polymorphism, Rachel must ensure that her subclasses pass the 'is-a' test, making them polymorphic and allowing her program to be more easily extended.
Step-by-step explanation:
When Rachel is working with subclasses and wants to employ polymorphism in her program, she must remember that a class that can pass the is-a test can be considered polymorphic. This concept is pivotal in object-oriented programming, allowing instances of a subclass to be treated as objects of a superclass. Hence, the ability to use a subclass object wherever a superclass object is expected. Polymorphism facilitates the design of a system that is easily extended and modified since objects can be replaced with instances of their subclasses without altering the correctness of the program.
When using polymorphism in a program, there are a few things Rachel should remember. First, if a class can pass more than one is-a test, it can be considered to be polymorphic. This means that the class can be treated as multiple types and can exhibit different behaviors depending on how it is used. This allows programs to be easily reused and changed.