Control Systems
A function of one or more variables which conveys information about the nature of physical phenomenon is called

 signal.
 interference.
 noise.
 system.

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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 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.
 at least one system is unstable and at least one system is causal.

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