单选题：Which of the following is the sufficient and necessary condition
Which of the following is the sufficient and necessary condition that an undirected connected graph has Euler circuit?
A.All the vertices have even degrees
B.At most 1 vertex has an odd degree
C.At most 2 vertices have odd degrees
D.Exactly 2 vertices have odd degrees
答案：A
A.All the vertices have even degrees
B.At most 1 vertex has an odd degree
C.At most 2 vertices have odd degrees
D.Exactly 2 vertices have odd degrees
答案：A
