please comment
I agree that #2 is also correct, in that the term "IS-A" is commonly used to describe a relationship in which the class in question "is a" subtype of another class (useful in
polymorphism). Because B implements A in the above example, B IS-A subtype of A and is usable wherever an instance of A is required (or at least *should* be, in principle - pun intended).
Do you have a question about this?