Is there a simple graph, each of whose vertices has even degree? Justify your answer.
a) Yes, because in a complete graph, every vertex has an even degree.
b) No, because in a simple graph, there cannot be a vertex with an even degree.
c) Yes, because in a bipartite graph, every vertex has an even degree.
d) No, because in a connected graph, the sum of the degrees of all vertices is always odd.