Radha was running

a. Due to strenuous exercise her muscles got fatigued as the accumulation of lactic acid in the muscles occurred because the muscles did not receive adequate oxygen during the exercise and by the anaerobic respiration pathway the glucose was broken down to lactic acid due to lack of oxygen.

b. The change in the breathing rate occurred because the body demands more oxygen during strenuous exercise for more ATP (i.e., Energy) production. Therefore, breathing rate increases to obtain more oxygen to produce more ATP by aerobic respiration.