用氮气作保护气体焊接铜时，熔池金属流动性降低，焊缝易产生（）。
A.裂纹
B.夹渣
C.气孔
D.未焊缝
参考答案：C
句型转换。
l. Li Hua is a very good football player. (改为同义句)
Li Hua _____ _____ _____ playing football.
Li Hua _____ football _____ .
2. I like Li Hua best,he is a football player. (改为同义句)
Li Hua is _____ _____ football player.
3. He wants to play basketball when he has time. (对划线部分提问)
____________________________________.
4. You look happy. (对划线部分提问)
5. He is tall and strong. (对划线部分提问)
土地的开发周期一般根据开发土地的面积规模和（）确定。
A．投资数额
B．开发的难易程度
C．资金来源
D．规划要求
