Will a fluoride mouthwash used after brushing reduce cavities? Twenty sets of twins were used to investigate this question. One member of each set of twins used the mouthwash after each brushing, the other did not. After six months, the difference in the number of cavities of those using the mouthwash was compared with the number of cavities for those who did not use the mouthwash. This experiment uses:

The correct option is matched pair design the reason for this is because in the experiment individual(elements) are group into pairs based on a characteristics that they have in common (in this question the characteristics being a twin) and then within each group the subject a placed on different treatment , in the case of this question each twin in each group are placed on different treatment (i.e one is given the mouthwash while the other is not ).

This process which I have explained is how a matched pair design is carried out