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问题 单项选择题

空气压缩机中间冷却器保安阀的开启压力为（），关闭压力≤300KPA。

A、450±20KPA

B、450KPA

C、380±20KPA

D、400±20KPA

答案

参考答案：A

解答题
(x+
1
2
)2+(x-
1
2
)2+(x+
1
2
)(x-
1
2
)．
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选择题

Everyone ______ very happy at the fashion show last Sunday. [ ]

A. is

B. are

C. was

D. were

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