正确答案: D

增大工件受力面积，减小压强

题目：用螺丝固定工件时，要在螺帽下垫一个面积较大的垫圈，使用垫圈是为了 【 】

解析：D

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学习资料的答案和解析：

[单选题]（ ）30． Many families were affected because of ________ ．

the broken banks of the river

解析：30.B【解析】从News 3可知。

[单选题]AMany children feel that the most important people in their lives are their friends．They be- lieve that their family members don-t know them as well as their friends． In large families， it is quite often for brothers and sisters to fight with each other and then they can only go to their friends for some ideas．

friends

解析：21.A【解析】从第一段前两句可知．

[单选题]20． The car is running _______' It seems to be flying．

faster and faster

解析：20．D【解析】由句意“这车开得越来越快，像要飞起来一样”．表示越来越快用“faster and faster"．故选D。

[单选题]The ducks ___________ many eggs last week．

laid

解析：B【解析】产蛋用lay，其过去式为laid。