What is bound to myosin when it is in its high-energy conformation?
I. Actin
II. ATP
III. ADP
(A) II only
(B) III only
(C) I and II
(D) I and III

The unit of contraction within skeletal muscle cells is called the sarcomere. A sarcomere contracts when the filamentous protein myosin stretches into a high-energy conformation and binds to the filamentous protein actin. When the myosin returns to its low-energy, relaxed conformation, actin is pulled, and the sarcomere contracts. The following steps relate ATP to each step of this process.
1- Myosin binds to actin (ADP is attached).
2- Myosin returns to low-energy conformation (ADP is released).
3- Myosin releases actin (ATP binds).
4- Myosin stretches to high-energy conformation (ATP is hydrolyzed).