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
Skeletal muscle contraction ends when: 
Select and explain. 
A. Ions move out of cytoplasm, B. Ach is absorbed from the NMJ, C. Closure and in­drawing of receptors, D. Decreased calcium outside reticulum
D: Decreased calcium outside reticulum
Answer: D: Decreased calcium outside reticulum 
Explanation: “Once the Ca2+ concentration outside the reticulum has been lowered sufficiently, chemical interaction between myosin and actin ceases and the muscle relaxes.”