Hi, I am Susan. I am from the U. K. But now I'm studying(学习) in China. I'm going to do many things in the winter holiday. First, I'm going to finish my homework. Then I'm going to take a trip. I'm going to Beijing. It's snowy in Beijing. There is white snow everywhere. So I'm going to make a snowman. I can ski. My parents will come to China with me. We are going to fly there. We are going to have a nice trip.

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下列山脉中，为长江水系与黄河水系分水岭的是（    ）

2006年我国自行研制的“枭龙”战机04架在四川某地试飞成功．假设该战机起飞前从静止开始做匀加速直线运动，达到起飞速度v所需时间t，则起飞前的运动距离为（　　）

如图所示，一个不记重力质量为m、带电荷量为q的粒子，从两平行板左侧中点沿垂直场强方向射入，当入射速度为v时，恰好穿过电场而不碰金属板．要使粒子的入射速度变为 $\frac{v}{2}$ 仍能恰好穿过电场，则必须再使（   ）