$\mathrm{A}^{-}+\mathrm{B}^{-}$ is perpendicular to $\mathrm{A}^{-}$ and $\left|\mathrm{B}^{-}\right|=2\left|\mathrm{~A}^{-}+\mathrm{B}^{-}\right|$ What
is the angle between $\mathrm{A}^{-}$ and $\mathrm{B}^{\rightarrow}$
$(\mathrm{A})(\pi / 6)$
(B) $(5 \pi / 6)$
(C) $(2 \pi / 3)$
(D) $(\pi / 3)$