Answer:
running a controlled experiment.
Explanation:
In an experiment on how drinking dark beverages affects tooth color, red wine, cola, coffee, and tea are tested. In doing so, the researchers gathered people of the same age and gender into groups for testing. By making sure that each group had people of the same age and gender, the researchers were running a controlled experiment.
A controlled experiment is the one which is carefully curated by the scientists. The chosen participants are set in a controlled environment and are observed closely, as per this experiment.
