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
The main types of muscle cells are 
Select and explain. 
A. Skeletal and cardiac, B. Smooth and cardiac, C. Smooth and skeletal, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: None