user: You are a structured reasoning expert tasked with solving problems step by step. 
You first think about the reasoning process in the mind and then provide the answer. The reasoning process and answer should be enclosed within <think> </think> and <answer> </answer> tags, respectively, i.e., <think> reasoning process here </think> <answer> answer here </answer>. 
Given a question and the associated knowledge graph triplets (entity, relation, entity), you are asked to answer the question with these triplets and your own knowledge. 
You can break down the original question into several sub-questions and explore the given knowledge graph to solve them step-by-step. If there are multiple possible answers, choose one as the final answer. 
After thinking, when you finally reach a conclusion, clearly state the answer within <answer> </answer> tags, i.e., <answer> answer here </answer>. 
Question: turbostar builds what class of locomotive? 
Knowledge Triplets: Turbostar, rail.locomotive_class.subclasses, British Rail Class 172
Turbostar, rail.locomotive_class.subclasses, British Rail Class 171
British Rail Class 170, rail.locomotive_class.maximum_speed, 160.9
Turbostar, rail.locomotive_class.maximum_speed, 160.9
British Rail Class 168, rail.locomotive_class.maximum_speed, 160.9
British Rail Class 172, rail.locomotive_class.parent_class, Turbostar
Turbostar, base.train.train_class_family.members, British Rail Class 171
Turbostar, base.train.train_class_family.members, British Rail Class 172
British Rail Class 171, rail.locomotive_class.parent_class, Turbostar
Turbostar, base.train.train_class_family.members, British Rail Class 168
Turbostar, rail.locomotive_class.subclasses, British Rail Class 168
Turbostar, base.train.train_class_family.members, British Rail Class 170
British Rail Class 168, rail.locomotive_class.parent_class, Turbostar
Turbostar, rail.locomotive_class.subclasses, British Rail Class 170
Turbostar, rail.locomotive_class.parent_class, British Rail Class 172
British Rail Class 171, rail.locomotive_class.maximum_speed, 160.9
British Rail Class 172, rail.locomotive_class.maximum_speed, 160.9
British Rail Class 170, rail.locomotive_class.parent_class, Turbostar

Answer: British Rail Class 171
British Rail Class 172
British Rail Class 170
British Rail Class 168