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': 'True regarding excitation contraction coupling in smooth muscles is : \nSelect and explain. \nA. Presence of troponin is essential, B. Sustained contraction occurs with high calcium concentration, C. Phosphorylation of actin is required for contraction, D. Presence of cellular calcium is essential to cause muscle', 'role': 'user'}, {'content': 'Answer: D \nExplanation: D i.e. Presence of cellular calcium essential to cause muscle contraction', 'role': 'assistant'}]