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

Sentence1: Booking was successful for Tuesday. Your Reference number is : 0J3V1091 . Sentence2: I'm sorry that doesn't seem to be available would you like me to try a different day or guesthouse? Sentence3: I'm sorry, there does not seem to be a historical site or hiking area in Cambridge. Is there anything else I can help with? Sentence4: Sure, I can help you with that. What section of town were you looking for? Sentence5: Doesn't really matter if they offer either of those or not. I just want it to be a 0 star hotel. Sentence6: Then the city centre north b and b is what you want, shall I book you? Sentence7: i don't really care about the area. Sentence8: I want to see local sights, can you help? Sentence9: I like hiking and seeing historical sites. Sentence10: Okay. Are you interested in hotels that offer internet and/or parking? Sentence11: Yes, I would like to find a hotel in the cheap price range? Sentence12: Yes, a guesthouse is fine. I was looking for zero stars and in the cheap price range. Sentence13: Great, thanks. I don't need anymore help. Goodbye! Sentence14: Yes, please book it for me. Sentence15: I'm sorry, I seem to be having an error in my system. You want a booking at the citycentre north B&B for 6 people, 5 nights, on Monday? Sentence16: I'm also looking for somewhere to go. Perhaps the college. Can you tell me the postcode for the college? Sentence17: City Cenrtree North and B is a cheap hotel,would you like me to book it for you ? Sentence18: Let book the same but on Tuesday instead. Sentence19: I'm happy I could help today. Goodbye! Sentence20: There are 18 colleges in town. I recommend christ's college in the centre. The postcode is cb23bu. Sentence21: Yes, there is a wide variety, do you have an activity type that your are interested in? Sentence22: Yes please, 6 people for 5 nights and we would be arriving Monday. Question: Who has spoken the sentence 21? (A) User (B) System
Answer: (B) System