Recall that in the Ames Test, potential mutagens are tested for their ability to revert His− auxotrophs to His+. Any particular his- mutation present in a His− auxotrophic bacterial strain would be able to be reverted only by some mutagens and not others. Yet, the purpose of the Ames Test is to determine whether or not a specific substance is a mutagen of any type. How is the Ames Test actually performed to circumvent this problem?

The authors of the paper "fudging the numbers: distributing chocolate influences student evaluations of an undergraduate course"† carried out a study to see if events unrelated to an undergraduate course could affect student evaluations. students enrolled in statistics courses taught by the same instructor participated in the study. all students attended the same lectures and one of six discussion sections that met once a week. at the end of the course, the researchers chose three of the discussion sections to be the "chocolate group." students in these three sections were offered chocolate prior to having them fill out course evaluations. students in the other three sections were not offered chocolate. the researchers concluded that "overall, students offered chocolate gave more positive evaluations than students not offered chocolate." indicate whether the study is an observational study or an experiment. give a brief explanation for your choice.