proof of Zermelo’s postulate

The following is a proof that the axiom of choice implies Zermelo’s postulate.

Proof.

Let $ℱ$ be a disjoint family of nonempty sets. Let $f:ℱ\to {\displaystyle \bigcup ℱ}$ be a choice function. Let $A,B\in ℱ$ with $A\ne B$. Since $ℱ$ is a disjoint family of sets, $A\cap B=\mathrm{\varnothing }$. Since $f$ is a choice function, $f(A)\in A$ and $f(B)\in B$. Thus, $f(A)\notin B$. Hence, $f(A)\ne f(B)$. It follows that $f$ is injective.

Let $C=\{f(B)\in {\displaystyle \bigcup ℱ}:B\in ℱ\}$. Then $C$ is a set.

Let $A\in ℱ$. Since $f$ is injective, $A\cap C=\{f(A)\}$.∎