Quiz: Cycle Detection in Directed Graphs

In directed graph cycle detection, you use $3$ colors: white (unvisited), gray (in current path), black (fully processed). You find a cycle when DFS encounters which color?

1. A.White. An unvisited node means you have gone in a circle
2. B.Gray. A node still on the current recursion stack means you have found a back edge
3. C.Black. A fully processed node means you are revisiting it
4. D.Any non-white color indicates a cycle