废金属的危害①____②____．

回收利用废金属的优点有____、____．

（2015•临沂）如图，在△ABC中，BD，CE分别是边AC，AB上的中线，BD与CE相交于点O，则$\frac{OB}{OD}$=____ 

以下有关家庭电路和安全用电的说法正确的是（　　）

我国公民行使国家权力、参与国家管理的一项最基本的政治权利是（  ）

假定英语课上老师要求同桌之间交换修改作文，请你修改你同桌写的以下短文。短文中共有10处错误，每句中最多有两处。错误涉及一个单词的增加、删除或修改。

增加：在缺词处加一个漏字符号(∧), 并在此符号下面写出该加的词。

删除：把多余的词用斜线（＼）划掉。

修改： 在错的词下划一横线, 并在该词下面写出修改后的词。

注意：1）每处错误及其修改均仅限一词；

2）只允许修改10处, 多者(从第11处起) 不计分。

    At a sports meet of high school I knew Christie and we became best friend. At that time, we often spend time together. Under her help, I made great progress in my study. Last year, she decided to study in abroad. In other words, we would be separated for long time. Before she leaving , I prepared a gift to show my best wishes to him. She said it was the best gift she has ever had. Until then, we've kept in touch with each other through e-mails. I look forward to see her again in the near future.