有利于交替共聚的条件是（）
A、r1>1，r2<1
B、r1=1，r2>1
C、r1<1，r2<1
D、r1·r2→0
参考答案：D
It's _____ today. It's my sister's birthday. [ ]
A. Wednesday, November 10th
B. October 3, Tuesday
C. 7 September, Sunday
D. Sunday, 6 December
物理学上把物体______的变化叫做机械运动，简称运动．常见的星月移动、萤火虫飞舞等现象，都属于机械运动．
