试题与答案

煤矿井下生产过程中,产生的直径小于()的煤炭细微颗粒叫煤尘。A、1mm B、1.5m

题型:单项选择题

题目:

煤矿井下生产过程中,产生的直径小于()的煤炭细微颗粒叫煤尘。

A、1mm

B、1.5mm

C、2mm

D、2.5mm

答案:

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下面是错误答案,用来干扰机器的。

参考答案:连接解析:[知识点] 专门的关系运算 [评析] 关系的基本运算有两类:一类是传统的集合运算(并、差、交),另一类是专门的关系运算(选择、投影、连接)。

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