06-b1-09A graph with an (undirected) edge between any two vertices is called a complete graph, and a complete graph containing n vertices is represented by $K_{n}$ Which of the following figures must not be a plan? 任何两个顶点间都有一条（无向）边的图称为完全图，包含n个顶点的完全图用$K_{n}$表示。下列哪个图一定不是平面图？
A: $K_{2}$
B: $K_{3}$
C: $K_{4}$
D: $K_{5}$
A: $K_{2}$
B: $K_{3}$
C: $K_{4}$
D: $K_{5}$
