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
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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
Which of the following is an effective opening for a first inquiry（）
A. Please answer the following questions about your Web hosting services
B. My name is Jill Aubrey, and I am the office manager for Cape Consulting Inc
C. I recently read an article about your firm in the Cape Cod Times
D. Cape Consulting Inc. is the premier communication-consulting firm in the Cape and islands area
What are the targets of drug action（）
A. Recipient
B. ion channel
C. enzyme
D. Endogenous small molecular matter
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