4. The number of trees with exactly 7 vertices (indistinguishable) is ( ). In graph theory, a tree is a connected graph without cycles, simply understood as a graph that connects \( n \) vertices with \( n-1 \) edges.
A. 9
B. 10
C. 11
D. 12
4. C As shown in the figure, use the exhaustive method.

We classify and list them in the order of the length of the longest chain in each tree:
Therefore, there are 11 trees that have exactly 7 vertices (undistinguished).

