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（∧），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限一词。
2）只允许修改10处，多者（从第11处起）不计分。
Last Sunday afternoon I went to a park by bike. I was walking in the park while a couple asked me if I could help take some photos of me. I was more than willingly to offer my service. With many thank, they waved goodbye to me. Therefore, a few minutes later, I found they had been dropped a black handbag on the bench where they had sat. Believe they would return, I decided to wait for them with the handbag on my hand. Soon, they hurried back and I returned the handbag to them, for that they thanked me again and again. What great pleasure it was to help others out!
纯水中所含氢离子的物质的量为
