PLC的工作原理，概括而言，PLC是按集中输入、集中输出，周期性（ ）的方式进行工作的。
A. 并行扫描
B. 串行扫描
C. 循环扫描
D. 一次扫描
查看答案
状态流程图包括（ ）部分。
A. 状态继电器、输入继电器、输出继电器
B. 状态继电器、有向连线、转移条件
C. 状态继电器、有向连线、转移条件、控制对象
D. 输入继电器、输出继电器
I. The man didn't want to travel because ______.
A. he was too old and in very poor health
B. he could not speak a foreign language
C. he thought he could learn everything about life without traveling
D. he preferred to stay home to enjoy his old age
2. The word "security" in the first paragraph might mean_____.
A. safety
B. care
C. danger
D. freedom
5. In Korea old people are happy because_____ .
A. they don't have to work any more
B. they can live a quiet and comfortable life
C. they can enjoy an honored and respected life
D. both b and c
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