Amina, a psychology researcher, conducts a study to find out whether having breakfast has an impact on people's performance at work. She chooses a sample of 100 participants and categorizes them into two equal groups: group A and group B. She asks group A to go to work after having breakfast and group B to go to work without having breakfast for 10 days. In the context of research designs, which of the following is true about this study?

a. Group A is the control group, and group B is the experimental group.
b. This study is classified as an ethnographic study.
c. This study is classified as a case study.
d. Group A is the experimental group, and group B is the control group.