1∈D, 1∈D, 1+ 1=2 does not belong to d, so d is wrong; So choose C.
5. If a loop passes through each side of the graph once and only once, it is called Euler path. Graphs with Euler paths are called Euler graphs.
Inference: An undirected graph G has Euler paths if and only if G is connected and all nodes have even degrees.
There are two 2-degree nodes and four 3-degree nodes in Figure A, so it is wrong.
Graph b has four 3-degree nodes, so it is wrong;
There are four 2-degree nodes and four 4-degree nodes in the C graph, which satisfies the inference, so C is selected;
D graph has 1 4-degree nodes, 2-degree nodes and 2-degree nodes, so it is wrong.