A discrete-time all-pass system has two of its poles at $0.25\angle 0^{\circ}$ and $2\angle 30^{\circ}$. Which one of the following statements about the system is TRUE?
- It has two more poles at $0.5\angle 30^{\circ}$ and $4\angle 0^{\circ}.$
- It is stable only when the impulse response is two-sided.
- It has constant phase response over all frequencies.
- It has constant phase response over the entire $\text{z-plane}$.