3. Recall that a directed acyclic graph (or "dag"), is a directed graph that does not contain any cycles. a. Prove that every finite dag has at least one vertex that has indegree zero (hint: you can use either proof by contradiction or proof by construction here). b. Show that the "finite" part of the previous question is necessary by describing how to construct an infinite dag in which all vertices have non-zero indegree. c. Since the property that the dag is finite is a necessary condition for the statement in part a to be true, there must be some part of your proof that specifically uses the fact that the graph is finite. Clearly identify where that is in your proof