If $z_{1}, z_{2}$ are two complex numbers such that $\left|\frac{z_{1}-z_{2}}{z_{1}+z_{2}}\right|=1$ and $t z_{1}=k z_{2}$ where $k \in \mathbb{R}$, then the angle between $\left(z_{1}-z_{2}\right)$ and $\left(z_{1}+z_{2}\right)$ is(A) $\tan ^{-1}\left(\frac{2 k}{k^{2}+1}\right)$
(B) $\tan ^{-1}\left(\frac{2 k}{1-k^{2}}\right)$
(C) $-2 \tan ^{-1}(k)$
(D) $2 \tan ^{-1}(k)$