设向量 $\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b},\stackrel{\rightharpoonup }{c}$满足 $\left|\stackrel{\rightharpoonup }{a}\right|=\left|\stackrel{\rightharpoonup }{b}\right|=1$, $\stackrel{\rightharpoonup }{a}·\stackrel{\rightharpoonup }{b}=-\frac{1}{2}$, $<\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{c},\stackrel{\rightharpoonup }{b}-\stackrel{\rightharpoonup }{c}>=60°$，则 $\left|\stackrel{\rightharpoonup }{c}\right|$的最大值等于（）

A．

2

B．

$\sqrt{3}$

C．

$\sqrt{2}$

D．

1