Topological order using bfs

Question

The following question was found in Sedgewick and Wayne book about algorithms in java:

4.2.19 Topological sort and BFS. Explain why the followin

Answer 1

You cannot use BFS, because a node with a higher rank may have an incident edge with a lower rank. Here's an example:

Let's say you start BFS at the source (A).

With the algorithm you proposed, node D would come before node C, which is clearly not a topological order. You really have to use DFS.