I split the class into two groups Each group is given a diff

I split the class into two groups. Each group is given a different interlocking metal puzzle to solve. I time how long it takes each group to solve their puzzle. My experimental design is... a. Within Subjects b. Mixed-Design c. Between Subjects d. Case-Study

It is a case study as you are calculating how much time it will take to solve different types of interlocking metal puzzle