向量 $|\boldsymbol{a}|=|\boldsymbol{b}|=1 ，|\boldsymbol{c}|=\sqrt{2}$ 且 $\boldsymbol{a}+\boldsymbol{b}+\boldsymbol{c}=0$ ，则 $\cos \langle\boldsymbol{a}-\boldsymbol{c}, \boldsymbol{b}-\boldsymbol{c}\rangle=$
$\text{A.}$ $-\frac{1}{5}$
$\text{B.}$ $-\frac{2}{5}$
$\text{C.}$ $\frac{2}{5}$
$\text{D.}$ $\frac{4}{5}$