This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
[{'content': "Indicator used in Horrock's apparatus produces which color in the presence of residual chlaine \nSelect and explain. \nA. Blue, B. Yellow, C. Green, D. Red", 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]