s7-1200PLC顺序功能图状态步的编号采用（）的一个位来表示。
A. 输入继电器I
B. 位存储器M
C. 顺序控制继电器S
D. 输出继电器Q
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All _____ is useful to us is good.
A. which
B. what
C. that
D. whether
Is this factory ____ we visited last week?
A. where
B. that
C. which
D. the one
she will never forget the day ____ she spend in Beijing.
A. when
B. what
C. which
D. why
This is the school _____ my father worked 20 years ago.
A. that
B. when
C. which
D. where
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