Stability

The behaviour of four dynamical systems S1, S2, S3 and S4 is shown in the following figures:

Response 1 Response 2
Figure 1: System S1 Figure 2: System S2
 
Response 3 Response 4
Figure 3: System S3 Figure 4: System S4

Which statements are correct ?

1 System S1 is stable. true false
2 System S3 is critically stable. truefalse
3 The characteristic equation of the system S4 has at least one root on the imaginary axis. true false
4 All roots of the characteristic equation of the system S2 must be in the right-half s plane. true false
5 The characteristic equation of system S3 has at least a root on the imaginary axis. true false