设向量 $\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b}$满足： $\left|\stackrel{\rightharpoonup }{a}\right|=3,\left|\stackrel{\rightharpoonup }{b}\right|=4,\stackrel{\rightharpoonup }{a}·\stackrel{\rightharpoonup }{b}=0$．以 $\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b},\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b}$的模为边长构成三角形，则它的边与半径为的圆的公共点个数最多为（）

A．

3

B．

4

C．

5

D．

6