— are you going to take?
—Books and clothes.[ ]
A. What
B. Why
C. Where
答案:A
65. We should roll _____ for the distinguished visitors in five-star hotel.
A.blue carpet
B.red carpet
C.green carpet
D.yellow carpet
如图所示,空间相距为的平行金属板加上电压后,它们之间的电场可视为匀强电场,其变化如图,当t=0时,A板电势比B板电势高,这时在靠近B板处有一初速度为零的电子(质量为,电量为)在电场力作用下开始运动,若要使这电子到达A板时具有最大的动能,则所加交变电压的频率最大不能超过多少?