A problem about IS-A

[Fu Dong Jia]

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.

[Kelvin Chenhao Lim]

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.

[Fu Dong Jia]

Thanks Kelvin!