问答题 根据中文提示，把对话中缺少的内容写在线上。这些句子必须符合英语表达习惯。打句号的地方，用陈述句；打问号的地方，用疑问句。
　　提示：Mary与Bill不期而遇，邀请他一起吃晚饭。但Bill当晚7点要去北京，下周一回来。Mary让Bill回来后给她打电话。
　　（Mary=M; Bill=B）
M: Bill, is that you? I haven’t seen you for a long time.
B: Hi, Mary.Great to see you again.
51　?
M: Fine.Why don’t we get together and have a chat? How about
52　this evening?
B: Well, it sounds great, but I’m
53　 at 7 o’clock.
M: What a pity!
B: Well, I’ll be back
54　.We can find a time to meet again.
M: Okay.
55　when you are back.
B: No problem.Goodbye!
M: Bye!（）