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
Cardiac muscle contracts due to 
Select and explain. 
A. Extracellular Ca2+, B. Sarcoplasmic Ca2+, C. Extracellular Na+, D. Intracellular Na+
A: Extracellular Ca2+
Answer: A: Extracellular Ca2+ 
Explanation: None