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  site  where  myosin  heads  bind  to actin  in  skeletal  muscles  are  covered by: 
Select and explain. 
A. Tropomyosin, B. Troponin, C. Calcium, D. None of the above
A: Tropomyosin
Answer: A: Tropomyosin 
Explanation: In the resting state, the tropomyosin molecules lie on top of the active sites of the actin strands, so that attraction cannot occur between the actin and myosin filaments to cause contraction.