Yоu expоse the bаcteriа Stаphylоcoccus aureus to low levels of the antibiotic methicillin. The surviving bacteria are then exposed to higher concentrations of methicillin and the process repeated until a strain of methicillin-resistant S. aureus (MRSA) was generated. To test your hypothesis, you perform an experiment by spreading the original strain of S. aureus and the MRSA strain onto agar plates containing doses of methicillin used in the hospital, and only the MRSA survives. Which of the following is the dependent variable in this experiment?