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 rate of skeletal muscle relaxation is related to the rate: 
Select and explain. 
A. At which free Ca++ is removed from the sarcoplasm, B. At which phosphocreatine is metabolized, C. Of ATP hydrolysis, D. Of acetylcholine resynthesis
A: At which free Ca++ is removed from the sarcoplasm
Answer: A: At which free Ca++ is removed from the sarcoplasm 
Explanation: The rate of skeletal muscle relaxation is related to the rate at which free Ca++ is removed from the sarcoplasm. A fraction of a second after its release Ca++ is pumped back into the sarcoplasmic reticulum. This removes Ca++ from the troponin thus allowing the tropomyosin to cover the active sites of the actin. This prevents the formation of cross bridges and causes relaxation. Acetylcholine is secreted at the neuromuscular junction and is necessary for initiation of action potentials at the muscle endplate. Phosphocreatine is a source of energy used to create ATP. ATP hydrolysis occurs in the myosin heads Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 5. Excitable Tissue: Muscle. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds),Ganong's Review of Medical Physiology, 24e.