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    Night after night，she came to help me sleep，even long after my childhood years．

    I don't remember when it first started making me a little angry-my mom's hands pushing my hair that way．But it really made me uncomfortable，for they felt rough against my young skin．Finally, one night，I shouted at her．"Don't do that anymore．Your hands are too rough!" She didn't say anything, but she never did it again．

    Years later, I missed my mother's hands and her goodnight kiss on my face. I was not a little girl anymore. My mom was in her mid—seventies, and her rough hands were still doing things for my family and me．   

    Now my own children have grown up．It was late on Thanksgiving Eve．As I slept in my bedroom，a familiar hand ran across my face to push the hair from my head. Then a kiss，ever so softly, touched my forehead．

    Taking my mom's hand，I told her how sorry I was for that night I shouted at her．But my mom didn't know what I was talking about．She had forgotten it for long．

    That night，I fell asleep with a new appreciation for my mother's caring hands. And the guilt that I had carried around for so long was nowhere to be found．

质量均匀分布的光滑球体C静止在如图所示的台阶和竖直墙壁之间，试画出球体C的受力示意图（O为球心）．

假定你是李华，你的美国朋友Jack来信说，他暑假后将到北京大学留学。请你根据以下内容给他写一封回信，告知相关情况。

1.简单介绍北京大学；

2.简单介绍北京（例如生活便利，居民友善等）；

3.留学期间愿意提供任何帮助。

注意：1.词数100左右：2.可以适当增加细节，以使行文连贯。

—8,844.43 meters. It's a popular place with mountain climbers.