Why Are You Able to Repeatedly Contract and Relax Your

Why are you able to repeatedly contract and relax your muscles of respiration, allowing you to breathe in and breathe out?

A) As soon as all of the Ca²⁺ stored in the lateral sacs of the sarcoplasmic reticulum is used up, muscle relaxation occurs.
B) After the muscle cell becomes excited, acetylcholinesterase rapidly destroys acetylcholine.
C) When there is no longer a local action potential in the muscle cell, Ca²⁺ is actively transported back into the lateral sacs of the sarcoplasmic reticulum, and muscle relaxation occurs.
D) Both (b) and (c) above.
E) All of these answers.