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': 'Calcium enters the cardiac cell during: \nSelect and explain. \nA. Rapid upstroke of the action potential, B. Down slope of the action potential, C. Plateau phase of the action potential, D. Slow diastolic depolarization (phase  4) of the action potential', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]