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Consider the feedback system shown in the figure. The Nyquist plot of $G(s)$ is also shown. Which one of the following conclusions is correct?

1. $G(s)$ is an all-pass filter
2. $G(s)$ is a strictly proper transfer function
3. $G(s)$ is a stable and minimum-phase transfer function
4. The closed-loop system is unstable for sufficiently large and positive $k$