【答案解析】Dear Susan, I am really upset to tell you that the book you had kindly lent me last week was lost yesterday. I read it every day and meant to finish it this week. so I took this book in my bag in order to finish it quickly. Yesterday I put my bag in the classroom and went to the washing room. Unfortunately, when I was back, my bag disappeared and it was the same to your book. I promise I will try my best to look for it. If I fail, I will buy a new one for you. Please excuse my carelessness. Yours sincerely, Jack