For a set of ordinals $A$, $\bigcup A$ is an ordinal

Question

I need to proove that for a set of ordinals $A$, the set $\bigcup A=\{x: \exists y \in A, x\in y \}$ is an ordinal as well. I would like some help with proofing that $ \forall x,y\in\bigcup A, x\in y \vee y\in x\vee x=y$. Thanks.