8。 A (Theorem has a connected planar graph, * * * has V nodes, E edges and R faces, then the formula: v-e+r=2 holds. )

9。 M-n+ 1 (if it is a spanning tree, it must be connected and has n- 1 edges, so subtract m-(n- 1) edges).

10。 B (Because it is a tree, it must be connected, with seven sides, and the sum of degrees is 14. We also know that the remaining five points share the remaining five degrees, so the degree of five nodes (leaves) is 1)