某个质量为m、带电量为﹣q（q＞0）的小球仅在重力作用下从静止开始沿竖直向下方向做匀加速直线运动，一段时间后在小球运动的空间中施加竖直方向的匀强电场，小球又经过相等的时间恰好回到出发点，则（   ）

个位、十位、百位、千位是四个计数单位。

    My parents want to take me to visit Beijing this summer holiday. I want to plan a good trip. So I must learn something about Beijing on the Internet. Beijing is a beautiful city. Autumn is the best season. There are many places of interest in Beijing. We want to visit the Great Wall, the Palace Museum and the Summer Palace. Beijing Duck is very famous all over the world. I hope I could have a try.

    We can get to Beijing by train. I want to buy a gift for my good friend. I also want to send a postcard to my teacher. I think I will have a great holiday.