PLEASE HELP ME IM TIMED!!

i think that is new experiments yield untrustworthy data and should be carefully considered, because you don't know what will happen with the experiments.

the answer is lactic acid from anaerobic respiration. the acid in lactic acid is what creates sore and burning muscles. if you think about how acid burns that will give you a clue for questions like this.

answer increasing the intensity, duration,and frequency of exercise progressively;