You are shown a conversation between a user and system. Identify who has spoken the indicated sentence based on the conversation.

Sentence1: I suppose, please give me a price and train ID. Sentence2: I would be happy to do that. I just need to know what day you would like your reservation to start. Sentence3: Which day would you like the reservation to start? Sentence4: Before we continue, I need the reference number for my hotel booking. Sentence5: The train id is TR1699 and the price is 12.80 pounds. How many tickets would you like to purchase? Sentence6: The closest train would be TR1699 departing Stevenage at 21:54 arriving Cambridge 22:43. The ticket would be 12.80 GBP. Shall I book this for you? Sentence7: 1 ticket for train ID TR1699 from stevenage to cambridge at 21:54 has been booked. Your reference number is W6J8TUAC . Have a nice day. Sentence8: Can you tell me the hotels that meet that criteria first? I need a 4-star guesthouse with parking. Sentence9: monday, please Sentence10: So is there any train on Monday? Sentence11: One ticket, please. Sentence12: No that will be all for today. Thank You! Sentence13: Are there any trains from stevenage to cambridge on Monday leaving at 21:00? That is what I'd prefer. Sentence14: It looks like there are only trains on Friday and Saturday from Stevenage. Would another day do? Sentence15: Hi I would like to find a place to stay that has a guesthouse and free parking. Sentence16: Also on Monday I am looking for a train destined for Cambridge. I need to depart at 21:00 from the Stevenage. Sentence17: Is there anything else I can do for you? Sentence18: Reference number is : DB9SPLT7 . Sentence19: Acorn Guest House has all the criteria you need. Would you like me to book that? Sentence20: Your booking was successful for 4 people at the Acorn Guest House, staying 2 nights starting Monday. Your reference number is : DB9SPLT7 . Is there anything else I can do today? Sentence21: Book it for 4 people staying for 2 nights and give me the reference number. Sentence22: There are two. One leaving at 21:54 and the other leaving at 23:54. Question: Who has spoken the sentence 21? (A) User (B) System
Answer: (B) System