Which statements are true(choose all that applay) a,is-a relationships always rely on inherritance. b,is-a relationships always rely on instance variables. c,is-a relationships always require at least two class types. d,is-a relationships always rely on Polymorphism. e,is-a relationships always tighthy coupled.

the answer is a,c and d.

I think there is only c is right. Can anyone explain this. thanks in advance.

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As you've discovered, this particular K&B mock exam question is unfortunately not very precise. See this thread for an earlier discussion of this question, including a brief comment by Bert.