在家庭电路中，火线和零线之间的电压是 ____V；图中哪些图所示情况会发生触电事故： ____．

红海从裂谷到海洋是因为哪两个板块张裂而成（  ）

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作文。文中共有10处语言错误，每句中最多有两处。每处错误仅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（Λ），并在其下面写出该加的词。

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。

    I have learned English for many years, but I don't have a real chance speak before foreigners. Last week, I happened to meet the foreigner. He badly needed my help and I felt so exciting. On Monday, I went to the flowers shop to buy a flower for my friend Mary, because she was ill. I was going to the hospital to see him. In the shop a foreigner and the boss was having a conversation, so the boss didn't know what the foreigner said. I hesitate for a while, then I came to help the boss. The boss and the foreigner thanked for me so much. I felt so happy though the language I learn is useful. Since then, I have been determined to learn English well.

在探究凸透镜成像规律的实验中，当蜡烛距透镜20cm时，光屏上得到清晰等大的像，则该透镜的焦距是____ cm．增大蜡烛与透镜的距离，为在光屏上再次得到清晰的像，应将光屏向____ 透镜（选填“靠近”或“远离”）的方向移动。