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Which one of the following is an eigen function of the class of all continuous-time, linear, time-invariant systems ($u(t)$ denotes the unit-step function)?

1. $e^{j\omega_0t}u(t)$
2. $\cos(\omega_0t)$
3. $e^{j\omega_0t}$
4. $\sin(\omega_0t)$