Quiz: DFS on Trees

You run DFS on a rooted tree and pass a parent parameter to avoid revisiting the parent. Why is this enough to prevent cycles in a tree, but not in a general graph?

1. A.Trees have directed edges, so you can only go one way
2. B.Trees have fewer edges, so collisions are rare
3. C.The parent parameter acts as a full visited array
4. D.Trees have exactly $1$ path between any $2$ nodes, so blocking the parent blocks the only back-edge