The open-loop transfer functions with unity feedback are given below for different systems.
1. \(\mathrm{G}(s)=\frac{2}{s+2}\)
2. \(G(s)=\frac{2}{s(s+2)}\)
3. \(G(s)=\frac{2}{s^{2}(s+2)}\)
4. \(\mathrm{G}(s)=\frac{2(s+1)}{s(s+2)}\)

Among these systems, the unstable system is
(a) 1
(b) 2
(c) 3
(d) 4