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: Anaesthesia
[{'content': "The most effective method to prevent hypotension during spinal anaesthesia is \nSelect and explain. \nA. Preloading with crystalloids, B. Metanephrine, C. Dopamine, D. Tredelenberg's position", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Here hypotension is due to block of sympathetic vasoconstrictor out flow to blood vessels resulting in venous pooling and a decreased return to hea.', 'role': 'assistant'}]