Researchers compared college students who reported they had smoked marijuana at least fifteen days a month with similar students who said they smoked five days a month at most. a day after going without marijuana, the heavy users performed significantly worse on tasks that involved sustaining and shifting attention. the research design is:

comparison-group                                        

Explanation:

Comparison group: In an experiment, the term comparison group refers to the group of the different unit that either receives an alternative treatment or no treatment at all. The aim of an experiment to involve a comparison group is that it is being used as a source of counterfactual causal inference.

The comparison group allows the experimenter to claim about the aspects of intervention without assigning the participants randomly.

In the question above, the research design is the comparison group.

answer; eliminate parking problems in cities, because a car took up only half as much space as a horse and buggy;