showed experimentally the possibility of electromagnetic wave propagation in waveguide（）
A. Heinrich Hertz
B. George Southworth and W.L.Barron
C. Oliver Heaviside
D. Lord Rayleigh
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A continuous signal is also called because its waveform is often analogous to that of the physical variable（）
A. an analog signal
B. a discrete signal
C. a digital signal
D. a sampled signal
Which of the following statements best describes the property of the system T if y1（t）=T[x1（t）], y2（t）= T[x2（t）] implies T[ax1（t-s）+bx2（t-s）]= ay1（t-s）+by2（t-s）（）
A. the system is linear
B. it is a linear time-invariant system
C. the system satisfies the principle of superposition
D. the system satisfies additivity and time-invariance
The unit impulse signal is the unit step signal（）
A. the same as
B. the differential of
C. the integral of
D. the cumulative sum of
After the negative feedback connection is adopted, the（）
A. closed-loop system must be stable
B. dynamic performance of the system will be improved
C. steady-state error finally be eliminated completely
D. structural parameters of the system need to be adjusted in order to improve the performance of the system
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