已知向量 a→,b→,c→\overrightarrow{a},\overrightarrow{b},\overrightarrow{c}a,b,c 满足 ∣a→∣=∣b→∣=1,∣c→∣=2\left| \overrightarrow{a}\right| = \left| \overrightarrow{b}\right| = 1,\left| \overrightarrow{c}\right| = \sqrt{2}a=b=1,c=2 ，且 a→+b→+c→=0→\overrightarrow{a} + \overrightarrow{b} + \overrightarrow{c} = \overrightarrow{0}a+b+c=0 ，则 cos⟨a→−c→,b→−c→⟩=\cos \langle \overrightarrow{a} - \overrightarrow{c},\overrightarrow{b} - \overrightarrow{c}\rangle =cos⟨a−c,b−c⟩= （ ）A. −45- \frac{4}{5}−54B. −25- \frac{2}{5}−52C. 25\frac{2}{5}52D. 45\frac{4}{5}54