当PH≤4.2时，水中碳酸化合物都是CO2，当PH≥8.3时，水中无CO2，但CO3

通过对机械危险的智能化设计，应使机器在整个寿命周期内发挥预定功能，包括误操作时，其机器和人身均是安全的，使人对劳动环境、劳动内容和主动地位的提高得到不断改善。这一特性称机器安全的( )。

A．系统性

B．友善性

C．防护性

D．整体性

根据句意，用括号中所给单词的适当形式或句子的语法要求填空。

1. Playing too many computer games is    ▲   (harm) to you.  

2. We should speak to the old man    ▲   (polite).    

3. —How many    ▲   (tooth) does a dog have?

—Sorry, I have no idea. Let’s search on the Internet.

4. A computer is    ▲   (expensive) than a TV set. 

5. Have you read    ▲   (today) newspaper? 

6. I    ▲   (not go) to bed until my mother came back at ten last night.

7. You need    ▲   (return) the comic book to the library at once.

8. My sister    ▲   (be) a singer since she left the school.

芯骨的设置应（）型芯通气槽的开挖。

钻孔使用冷却液的目的是（）

A.为了快速完成钻孔工序

B.使钻头切削部分降温

C.使钻头切削部分润滑

D.提高孔壁光洁度

生物钟