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　  Do you carry too much on the way to school or home? Don’t worry．Try e-readers (电子阅读器) and say goodbye to your heavy schoolbag．
   An e-reader has the memory space to hold hundreds of books．So people want to (A)扔掉沉书包 and read with small, light e-readers．For example，Cushing Academy，a high school in Massachusetts，USA，replaced (替换) most of the 20，000 books in (B) its library last summer．Teachers and students will use e-readers instead．
   Most e-readers are thin and weigh less than 500g．They can download (下载) an e-book in 60 seconds．“It’s (C) so easy．You can have a lot of books right at your fingers!” said Meghan Chensusky．
   Chinese schoolchildren are also trying e-readers．At the Shanghai World Expo (世博会)，an e-schoolbag is now on show．It is a special e-reader．Students can use it to download textbooks from the school servers (服务器)．(D)It can also send and receive homework．Some Schools in Beijing and Shanghai have tried this e-schoolbag for a few years．“It’s excellent．I (E)don’t have to carry a heaw schoolbag anymore，”said Xiao Qi，15，Beijing．
   New e-readers are not just for reading．They have Wi-Fi and multimedia (无线电视和多媒体)．Parents worry that these may hurt their children’s study habits． “I think Wi-Fi is not necessary for kids，”said Zeng Wenwen，43， Beijing．

下列现象中，由动能转化为重力势能的是(     )

在平整地面上有一层厚度均匀的积雪，小明用力向下踩，形成了一个下凹的脚印，如图所示，脚印下的雪由于受外力挤压可近似看成冰层，利用冰的密度，只要测量下列哪组物理量，就可以估测出积雪的密度？（　　）

泊秦淮

杜牧

烟笼寒水月笼沙，夜泊秦淮近酒家。

商女不知亡国恨，隔江犹唱后庭花。