A:walk
B:to walk
C:walking
D:walked
天气日历是记录每天各种天气现象的( )。
When he was running after his brother, the boy lost his ___ and had a bad fall.
如图所示,两个相同的半圆形光滑绝缘轨道分别竖直放置在匀强电场E和匀强磁场B中,轨道两端在同一高度上,两个相同的带正电小球a、b同时从轨道左端最高点由静止释放,且在运动中始终能通过各自轨道的最低点M、N,则( )