Algo-Topological Sort

Question

Answer 1

For topological sort find vertices(with edges out from that vertex) which have 0 in- degree. if Dircted graph contain cycle then no topological sort possible,

A is answer D,A,B,F,E,C

many other order are possible. like D,A,C,B,E, and A,D,C,B,E,F etc

Comments

D should be always followed by C or E right ? How can D,A,B,F,E,C be a topological sorting Order ?

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Here $D\rightarrow E$ or$D\rightarrow C$ it means $D$ must come Before $E$ or $C$.

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How many number of topological sort are possible on above graph.