Answer:
C. lactic acid
Explanation:
"Lactic acid" build-up in the muscles occur when a person does an "anaerobic exercise," specifically high-intensity ones. Due to lack of oxygen going to the muscles (temporary), "glycolysis" occurs. This means that glucose will be used in order to provide the energy needed. This process produces lactic acid as an end-product. This makes the muscles "sore," thus, it is important to have rests per week in order to eliminate lactic acid.
