Consider the following directed graph: Which of the following is/are correct…
2021
Consider the following directed graph:

Which of the following is/are correct about the graph?
- A.
The graph does not have a topological order.
- B.
A depth-first traversal starting at vertex S classifies three directed edges as back edges.
- C.
The graph does not have a strongly connected component.
- D.
For each pair of vertices u and v, there is a directed path from u to v
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Correct answer: A, B
Final conclusion: The only correct statement is that the graph does not have a topological order.
Reason:
Presence of directed cycles prevents a topological ordering. A topological order exists only for directed acyclic graphs (DAGs). In this graph, following the directed edges around any small directed square brings you back to the starting vertex, so a directed cycle exists.
Depth-first search edge classification depends on the neighbor visitation order. The exact number of back edges cannot be determined from the picture alone without specifying how neighbors are visited; each directed cycle contributes at least one back edge in some DFS orders, but the total count may vary.
There are strongly connected components: nodes on the same directed cycle are mutually reachable, so they form nontrivial strongly connected components. Therefore the claim that there are no strongly connected components is false.
The graph is not strongly connected as a whole, so it is not true that for every pair of vertices u and v there is a directed path from u to v. Edge directions restrict reachability between some vertices.
Therefore the correct answer is: the graph does not have a topological order, because directed cycles are present.
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