Control Systems
Lowest critical frequency is due to pole and it may be present at the origin or nearer to the origin, then the type of network is

 RC.
 LC.
 RL.
 Any of the above.

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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.
 each system in the cascade is individually caused and unstable.
 the majority are unstable and the majority are causal.

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