It's not proper for kids to have too much pocket money because they may buy ______.
A:something necessary
B:necessary anything
C:something unnecessary
D:unnecessary something
大、小两圆同心,环形面积是小圆面积的ab倍,若大、小两圆的半径分别为R、r,则Rr等于( )
如图所示,一半径为R的均匀带正电圆环水平放置,环心为O点,在O正上方h高位置的A点与A′关于O对称.质量为m的带正电的小球从A点静止释放,并穿过带电环.则小球从A点到A′过程中加速度(a)、重力势能(EpG)、机械能(E)、电势能(Ep电)随位置变化的图象一定不正确的是(取O点为坐标原点且重力势能为零,向下为正方向,无限远电势为零)( )
—Do you have bags? —No, I don't. But I have six _____.