You are shown a conversation between a user and system. Identify who has spoken the indicated sentence based on the conversation.
Q: Sentence1: yes book it for 4 people Sentence2: Yes sure. I have a reference number for you and it is 33TISTQB . Sentence3: Om I have train id TR1791 that fits your needs. Shall I book it ? Sentence4: Train TR1791 will leave at 17:00 on Thursday. It will cost 94.4GBP. Your reference number is HN91KJWK . Have your luggage ready, thank you! Sentence5: Is there a certain type of cuisine or part of town you're looking for? There are 22 restaurants to choose from. Sentence6: anytime after 17:00 on thursday going to london kings cross Sentence7: Thank you! Have a great day! Sentence8: I am traveling to Cambridge and looking forward to try local restaurants. Sentence9: I am departing from cambridge and want to arrive in london kings cross. Sentence10: Yeah. I'd like a moderately priced tuscan food restaurant. Sentence11: Thank, that is everything i need, you were so helpful. Sentence12: Thanks so much. Can you help me find a train? Sentence13: We have 5 different train times for you all on Friday. What time would you like to leave? Sentence14: Just so I can narrow down the results and get you exactly what you need, what's your departure station? Sentence15: I'm sorry there's no matching results. Sentence16: Great can you book that for 4 people at 17:45 on sunday? Sentence17: Okay, can we try modern European food instead, please? Sentence18: How about Galleria at 33 Bridge Street in the town centre. Great food. Question: Who has spoken the sentence 15? (A) User (B) System
A: 
Answer: (B) System