往复式泵是什么泵的一种？说出它的工作原理？

短文改错（共10小题；每小题1分，满分10分）
假如英语课上老师要求同学们交换修改作文，请你修改你同桌写的作文。文中共有10处语言错误，要求你在错误的地方增加，删除或修改某个单词。
增加： 在缺词处加上一个漏字符号 （ ∧ ）， 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， 并在该词下面写上修改后的词。
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11处起 ） 不计分。

例如： It was very nice to get your invitation to spend ∧ weekend with you. Luckily                                        the I was completely free then, so I’ll to say “yes”. I’ll arrive in Bristol at around 8 p.m. am in Friday evening. on

    Recently，our school has held the meaningful discussion on whether it is reasonable for middle school students to have classes on weekends．Some of our classmates believe we should have classes on weekends．because we can make fully use of the free time to review the lessons we had learnt．Some others are against the opinion，saying we had better to have a good rest on weekends so that we can work effective during the weekdays．The rest agree with neither of the opinion．They have the view which keeping the balance between work and play are both important for us．They suggest one day of the weekend should used to learn our lessons and during the other day they do something interested or have a rest．