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The Thevenin equivalent impedance $Z_{\text {th }}$ between the nodes $P$ and $Q$ in the following circuit is

1. $1$
2. $1+s+\frac{1}{s}$
3. $2+s+\frac{1}{s}$
4. $\frac{s^{2}+s+1}{s^{2}+2 s+1}$