6. Given $\sin \alpha+\sin \beta=1, \cos \alpha+\cos \beta=0$, then the value of $\cos 2 \alpha+\cos 2 \beta$ is
(A) 1
(B) $\frac{3}{2}$
(C) $\frac{2}{3}$
(D) $\frac{3}{4}$
A

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