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

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

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Control Systems
In the integral control of the single area system frequency error is reduced to zero. Then

  integrator output and speed changer position attain a constant value.
 integrator o/p decreases but speed changer position moves up.
 integrator o/p increases but speed changer position comes down.
 integrator o/p decreases and speed changer position comes down.

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