①CO2，②CO，③CH4三种含碳的物质，按所含碳元素的质量分数由高

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问题填空题

①CO₂，②CO，③CH₄三种含碳的物质，按所含碳元素的质量分数由高到低排列顺序排列为______（填序号）．

答案

①CO₂的相对分子质量是44，②CO的相对分子质量是28，③CH₄的相对分子质量是16，

含碳元素的质量分数分别是①

×100%≈31.8%，②

×100%≈42.9%，③

×100%=75%，

所以按所含碳元素的质量分数由高到低排列顺序排列为③，②，①．

故答案为③，②，①．

补全对话，情景问答

Complete the dialogue with proper words or sentences.

M-Michael L-Lin Tao

L: What are you going to do this Sunday?

M: (1) ___________________________________. What about you?

L: I don't know, either. Would you like to do something interesting with me?

M: Sure, what would you like to do?

L: (2) ___________________________________.

M: OK. Where are we going to meet?

L: Let's meet at my uncle's house. My uncle has a boat. He can take us to fish on the lake.

M: (3) ___________________________________?

L: Let's leave at four in the morning. Fishing is good at that time.

M: (4) That's too early. ___________________________________?

L: Well, no problem. What about half past four?

M: (5) _________________________________. See you at half past four on Sunday morning.

单项选择题

焊前清除坡口边缘的目的是（）。

A.增加熔深

B.减少焊缝金属中氢含量

C.防止焊缝产生咬边和夹渣

D.防止热裂纹