由于纯铜导热率高，所以气焊时，火焰能率应( )。
A. 比低碳钢小1～2倍
B. 比低碳钢大1～2倍
C. 和低碳钢相同
D. 比低碳钢大3～4倍
铝及铝合金焊丝根据化学成分可分成纯铝、铝镁、铝铜、铝锰及铝硅几大类。（）
A. 对
B. 错
From the passage we may come to the conclusion that ______.
A. joining big companies are more beneficial to graduates
B. smaller companies are the best choice for graduates
C. graduates should listen to their parents' opinions when choosing a job
D. both big companies and small ones have their own advantages and disadvantages
People have different opinions about ______.
A. why the concentration of gas is rising steadily
B. to what degree the greenhouse effect can affect us
C. whether CO2 has anything to do with the greenhouse effect
D. what measures should be taken to lower the carbon dioxide levels
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