Given the task definition and input, reply with output. You are shown a conversation between a user and system. Identify who has spoken the indicated sentence based on the conversation.

Sentence1: I've made those reservations and your reference number is N90KBZUM Sentence2: Thanks a lot. That is all I need. Sentence3: Train TR2232 leaves Liverpool Street, London at 17:39 and arrives at 19:07. Would this work for you? Sentence4: how about TR9886? it arrives at 19:07 on Friday. Sentence5: Is it expensive? Sentence6: Yes that works. Go ahead and book me 7 tickets please Sentence7: Your booking was successful and your total cost is 116.2 GBP. Your reference number is BWXB511A . Sentence8: ashley hotel is a moderately price hotel in the north area. Would you like me to book a room for you? Sentence9: I would prefer one in the expensive range if you have any. Sentence10: Great thank you so much. Sentence11: ashley hotel is a great place with free wi fi Sentence12: The express by holiday inn is an expensive 2 star hotel in the east. They have free parking and free wifi. Will this work for you? Sentence13: Thanks again, goodbye. Sentence14: Yes. I need to book for 7 people starting on Wednesday and staying for 2 nights. Sentence15: Great! Can you please help me with finding a train as well? I need something leaving london liverpool street after 16:30. Sentence16: Have a great trip. Goodbye. Sentence17: Actually, can we try for Wednesday? I'll need tickets for the same group of people. Sentence18: I'm looking for a hotel that includes free wifi. Question: Who has spoken the sentence 15? (A) User (B) System
Answer: (B) System