Prove that A* is the smallest reflexive and transitive set containing A I'm trying to learn automata theory on my own and I am running into an issue with the second part of the question:

We say B is transitive if $BB\subseteq B$ and reflexive if $\epsilon \in B$


  Show that A* is a reflexive and transitive set containing A and if B is any other reflexive and transitive set containing A, then $A^*\subseteq B$.


I've shown that Kleene star satisfies these two conditions. I've tried partitioning B into two sets with $A = B \cup C $ and trying a constructive proof but this hasn't led any where. I also am considering a proof by contradition but don't know where to start.

Can you help me with a hint on how to approach this problem?
Hint: Try the usual approach to show set inclusion! That is, pick $w \in A^*$ and show that $w \in B$.

Next hint:


   Apply transitivity of $B$ repeatedly.


More elaborately:


   If $w \in A^*$ then either $w = \varepsilon \in B$ or there are $w_1, \dots, w_k \in A \subseteq B$ with $w = w_1 \cdot \dots \cdot w_k$. Now, because $BB \subseteq B$ we have $w_2' = w_1 w_2 \in B$. For the same reason, $w_2' w_3 \in B$. And so on. Perform induction for the general proof.