A researcher is trying to determine whether a new painkilling medicine is effective in reducing headaches. Twenty adult participants sit in a noisy environment for 30 minutes and then rate the severity of their headaches on a scale from 0 (none) to 5 (intense). Group M (the medicine group) receives a normal dose of the medicine. Group C (the control group) rests quietly. After 20 minutes, all participants again complete the headache rating scale. The statistically significant differences in headache severity on the posttest appear in the graph below.
Which of the following statements is supported by the results of this experiment?