下列气体中,不能用排空气法收集的是( )
A.H2
B.CO2
C.NO
D.O2
答案:C
一氧化氮气体遇空气,会迅速化合生成二氧化氮气体;
物体B放在物体A上,A、B的上下表面均与斜面平行,当两者以相同的初速度靠惯性沿光滑固定斜面C向上做匀减速运动时,下列说法正确的是
A.A受到B的摩擦力沿斜面方向向上
B.A受到B的摩擦力沿斜面方向向下
C.A、B之间的摩擦力为零
D.A、B之间是否存在摩擦力取决于A、B表面的性质
同义句变化。
1. In the tree there is a bird.
A bird _____ in the tree.
2. Ann isn't wearing a red sweater.
Ann isn't _____ a red sweater.
3. Li Lei is a friend of mine.
Li Lei is _____ _____.
4. Mr. Chen is at work now.
Mr. Chen _____ _____ now.
5. Are we all here today?
_____ _____ here today?