The independent variable in an experiment is the variable that the experimenter manipulates or changes in order to effect a measurable response in the dependent variable.
According to this question, an experiment is conducted to determine what concentration of disinfectant is most effective in killing bacteria. The independent variable is the CONCENTRATION OF DISINFECTANT because it is the variable being changed or manipulated.
Exercise causes the heart to pump blood into the circulation more efficiently as a result of more forceful and efficient myocardial contractions, increased perfusion of tissues and organs with blood, and increased oxygen delivery.
