Possible Duplicate:
Using a choice function to find an inverse for $F\colon A\to P(B)$
Let $F:A \rightarrow \mathcal P (B)$ be arbitary functions which covers $B$.
Use AC to show there is a function $\phi: B \rightarrow A$ such that $b \in F( \phi(b))$ for each $b \in B$ .
Do I use AC on the set B. I don't see what I'm choosing here.
For every $b\in B$ there is some $a\in A$ such that $b\in F(a)$. You need to choose such $a$ for every $b$.