A:14.0%
B:15.4%
C:33.4%
D:37.2%
— Twice a week. We clean their rooms and do some shopping for them.
如图甲,一木板A放在水平桌面上现用一水平力F,向右缓慢地推木板,将其推到桌子中间,如图乙所示,在推木板的过程中木板对桌面的压力F、摩擦力f的变化情况是( )
如图,直线AB、CD相交于点E,DF∥AB. 若∠D=70°,则∠CEB等于( )