1. Let (S,d) be metric space and $\displaystyle A \subset S$. Then frontier A equals:
A.
B.
C.
D.

2. (S,d) is metric space, $\displaystyle A \subset S$. A' is:
A.
B.
C.
D.
E.
F.

Please, help me to answer and explain.
Greetings, thanks in advance!