Control Systems
In polar plots if the critical point '-1+j0' is enclosed then the system is --------

 unstable
 stable
 marginally stable
 critical stable

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Control Systems
A cascade of three linear time invariant systems is causal and unstable. From this we conclude that

 at least one system is unstable and at least one system is causal.
 at least one system is causal and all systems are unstable.
 the majority are unstable and the majority are causal.
 each system in the cascade is individually caused and unstable.

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