A research group interested in understanding the effect of listening to classical music on learning mathematics conducted a study. One hundred students were randomly assigned to one of two groups. The first group listened to classical music as they learned a new lesson. The second group learned the same lesson with no music playing. Both groups were tested on the new material to determine how much they learned. The results for the two groups were compared. What kind of study is this?

This is an observational study because students were randomly assigned to groups.
This is an observational study because a treatment was applied to both groups.
This is an experiment because a treatment was applied to a group.
This is an experiment because students were able to choose which group to join.