Standing in Line, Waiting to Board a Bus, You Leave

Standing in line, waiting to board a bus, you leave your backpack on the ground. Deciding to move your backpack you pick it up and set it down close by. Your biceps and other muscles contract when you lift up the backpack and relax when you set it down. Which of the following is the most likely reason that the biceps extended back to its original length?

A) the weight of the backpack extended the biceps when the biceps relaxed
B) calcium was returned to the sarcoplasmic reticulum, allowing myosin heads to push the actin fibers apart
C) when myosin heads detach from actin filaments, they help to extend the actin back to its extended length
D) tropomyosin, a regulatory protein, binds to exposed myosin-binding sites causing actin filaments to uncoil and extend back to their original uncontracted lengths.