Let A ⊂ R . Can any of the following hold?

(a) ∂A = ø (b) ∂A = A (c) A ⊂ ∂A and A not equal to ∂A

I do not know how to get the answers to the above. I know that the boundary of a set is empty if the boundary is clopen but I was always taught that boundary is closed. And as I am not given a value for A I get really confused Please help.