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