If $z_1,z_2$ are two complex numbers such that $\large|\frac{z_1+z_2}{z_1-z_2}|=1,$ then $\large\frac{z_1}{z_2}$ is ?

Question

$\begin{array}{1 1}(A) positive\; real \\(B) Negative \;real \\ (C) cannot\; say \\ (D) Zero\; or\; purely\; imaginary. \end{array}$