The correct answer is A.
Muscles need energy to produce contractions. The energy is derived from ATP (which stands for adenosine triphosphate) that is present in muscle cells.
Muscles tend to contain only limited quantities of ATP. When depleted, ATP needs to be resynthesized from other sources namely creatine phosphate (CP) and muscle glycogen. Other supplies of glycogen are stored in the liver. The body is also able to resynthesize ATP from lipids, i.e. free fatty acids. Different sources of energy are used depending on intensity and duration of the workload .
