Muscle physiology- if injury was responsible for the development of soreness

In a fitness test a subject repeatedly stepped up using the right leg while the left leg was used to oppose the effect of gravity in the step down. Muscle pain was experienced several hours after the test. The soreness reached a peak in 1 to 2 days and persisted for about a week. The presences of proteins specific for muscle in the subject’s serum provided evidence for muscle cell injury. Structural damage was apparent in biopsy samples from the affected muscles.

I know the answer is C but I am not sure why

1.  If injury was responsible for the development of soreness, which muscles would be
      most affected? Explain your answer.

      A. All the muscles in both legs.
      B. All the muscles that contracted during the fitness test.
      C. The muscles that contracted in the left leg.
      D. Primarily the muscles in the right leg that were actively shortening.
      E. The relaxed muscles that were passively stretched.