A researcher studies the number of deaths caused by a new disease in a county for 2 years before releasing a new medicine that could potentially cure it, and 2 years after releasing the medicine. During the same period, he measures the number of deaths caused by the disease in six other counties where this new medicine was not introduced. This research design is an example of a(n) a. control series design b. Solomon four-group design c. posttest-only design d. interrupted time series design