题目内容

以下是一些系统函数的收敛域,则其中稳定的是()

A. |z| < 0.9
B. 0.5 < |z| < 2
C. |z| > 2
D. |z| < 0.5

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下列序列中z变换收敛域包括|z|=∞的是()

A. u(n)+u(n+1)
B. u(n)-u(n-1)
C. u(n)-u(n+1)
D. u(n+1)-u(n)

Paper from the wood.

A. make
B. made
C. is made
D. makes

Great changes in the past 20 years.

A. have taken place
B. takes place
C. are taken place
D. is taken place

Young children must by adults.

A. look after
B. is looked after
C. looked after
D. be looked after

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