Полевой эксперимент

To conduct clinical trials of a new medication, patients are randomly divided into two groups

To see which textbooks are more efficient to study math, the government obliges half of the country’s schools to use textbook A and the other half to use textbook B

Different regions of a country may have different fines for speeding, so one can study how changes in fines affect drivers’ behavior

None of the other options is correct