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: Physiology
[{'content': 'ABO antigens are not present in ? \nSelect and explain. \nA. Saliva, B. CSF, C. Blood, D. Tear', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. is \'b\' i.e., CSF"Soluble ABO antigens are detected in secretions and all body fluids except cerebrospinal fluid".', 'role': 'assistant'}]