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