The equation $\left|z-z_{1}\right|^{2}+\left|z-z_{2}\right|^{2}=k, k \in R$ represents a circle if
(A) $k \geq \frac{1}{2}\left|z_{1}-z_{2}\right|^{2}$
(B) $k \leq \frac{1}{2}\left|z_{1}-z_{2}\right|^{2}$
(C) $k \geq \frac{1}{2}\left|z_{1}+z_{2}\right|^{2}$
(D) $k \leq \frac{1}{2}\left|z_{1}+z_{2}\right|^{2}$