When You Pick Up Your Backpack and Sling into onto Your

When you pick up your backpack and sling into onto your shoulder, your biceps contract. What must occur for your arm to move back down to your side as you walk away?

A) An antagonistic muscle must contract.
B) The tendon connected to the biceps must elongate.
C) No muscles must contract.
D) Not only must an antagonistic muscle contract, but also the tendon connected to the biceps must elongate.