Control System Power Reduction, Stability Needs, Natural Frequency, Z Transform ROC

Q.1: To decrease the power in control system we use :
Open loop
Close loop
Combination of the these two with active elements
Combination of the these two with passive elements

Q.2: Which system needs more stability
A. Automatic washing machine
B. Traffic signal system
C. Home heating system
D. Motor with position control

Q.3: Natural frequency in the control system represents what ?

Q.4: If z transform lies in ROC then:
A. System is stable
B. Inverse of system is stable
C. Both system and its inverse are unstable
D. Both system and its inverse are stable

Hi!,

q.1 closed loop
q.2 a,b,c are examples of open loop systems.
q.3 Natural frequency is the frequency at which the system would oscillate if the damping factor were zero.
q.4 System is stable

If I were to answer the questions, that would be my answer..
Best,
Kevin