Consider these questions:
If course A requires B, and B requires A, can you finish both?
What graph property makes a valid course schedule impossible?
Which technique detects this property? Try to connect this to what you have learned. Next unit reveals the pattern.
A cycle in the prerequisite graph means a group of courses that all depend on each other. No matter which one you try to take first, it requires another from the group. Cycle detection through topological sort or DFS coloring solves this.