设向量 $\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b}$满足 $\left|\stackrel{\rightharpoonup }{a}\right|=\left|\stackrel{\rightharpoonup }{b}\right|=1$, $\stackrel{\rightharpoonup }{a·}\stackrel{\rightharpoonup }{b}=-\frac{1}{2}$,则 $\left|\stackrel{\rightharpoonup }{a}+2\stackrel{\rightharpoonup }{b}\right|=$（）

A．

$\sqrt{2}$

B．

$\sqrt{3}$

C．

$\sqrt{5}$

D．

$\sqrt{7}$