齿轮油要求有良好的润滑性和（），低凝点。
A、稳定性
B、抗磨性
C、乳化
D、保温
参考答案：B
补全下面的对话。
It is Saturday afternoon. Sam and Amy are going to cinema by bike. (At a crossing)
Sam: S 1 , Amy! The light is red.
Amy: Oh, sorry. I didn't notice it.
Sam: Don't h 2 , Amy. Wait! The light is y 3 . We must w 4 .
Amy: Look! The light is green now. Let's go.
Sam: All r 5 .
如图13-5-8所示，光滑绝缘细杆竖直放置，它与以正点电荷Q为圆心的某一圆周交于B、C两点，质量为m，电荷量为-q的有孔球从杆上的A点无初速下滑，已知q≤Q，=h.小球滑到B点时速大小为.求：
图13-5-8
（1）小球由A到B的过程中电场力做的功；
（2）已知Ub=Uc，求A、C两点间的电势差.
=h.小球滑到B点时速大小为
.求：