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': 'Cardiac muscle contracts due to \nSelect and explain. \nA. Extracellular Ca2+, B. Sarcoplasmic Ca2+, C. Extracellular Na+, D. Intracellular Na+', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]