java IS-A relationship exam question confusion

Question

From MasterExam:

Which statements are true? (Choose all that apply)

A. is-a relationship always rely on inheritance
B. is-a relationship always rely on instance variables
C. is-a relationship always require at least two class types
D. is-a relationship always rely on polymorphism
E. is-a relationship are always tightly coupled

Correct answers: A, C, D

I don't see how any of A, C or D are correct.

An Object object IS-A Object. A String object IS-A String. There is only one class type in each of these statements and no inheritance or polymorphism applies.

Is my rationale wrong or are the answers incorrect?

Answer 1

"Relationship" refers to a relationship between two classes. An is-a relationship is a type of relationship that uses inheritance (as opposed to e.g. has-a, which uses composition). For instance, a String is-a Object. A class can't inherit from itself, which implies C. As a side note, a class could have composition (has-a) with itself. E.g. a Person could have another Person as a mother field.

Any time you have inheritance, an instance of the subclass can be used as an instance of the superclass. That's polymorphism, which means D is also right.