地球是人们赖以生存的环境,保护地球是我们的责任。请问地球上最大的生态系统是[ ]
A.农田生态系统
B.草原生态系统
C.湿地生态系统
D.生物圈
答案:D
读一读,重新排序,使对话通顺。
A. Yes, here you are.
B. I'm going to buy some storybooks.
C. Can I help you?
D. Yes. Do you have some storybooks?
E. What are you going to buy, Sarah?
正确顺序是:_________________________________
如图所示,固定斜面上除AB段粗糙外,其余部分光滑,物块与AB段间动摩擦因数处处相同。当物块从斜面顶端滑下后,经过A点的速度与经过C点时的速度相等,且AB=BC.已知物块通过AB段和BC段所用时间分别为t1和t2,动量变化量分别为△p1和△p2,则( )
A.t1=t2, △p1=△p2 B.t1>t2, △p1=△p2
C.t1<t2, △p1=△p2 D.t1=t2, △p1=-△p2