To explore the effects of multivitamin supplements on health, you recruit 100 volunteers. Half are to take a multivitamin supplement daily. The other half are to take a placebo daily. Multivitamin supplements may have different effects on men and women because their nutritional needs may differ. Forty of the volunteers are women and 60 are men, so you separately randomly assign half the women to the multivitamin group and half the men to the multivitamin group. The remaining volunteers are assigned to the placebo group. This is an example of O an observational study. O a completely randomized design. O a matched pairs design. O a block design but not matched pairs. To explore the effects of multivitamin supplements on health, you recruit 100 volunteers. Half are to take a multivitamin supplement daily. The other half are to take a placebo daily. Multivitamin supplements may have different effects on men and women because their nutritional needs may differ. Forty of the volunteers are women and 60 are men, so you separately randomly assign half the women to the multivitamin group and half the men to the multivitamin group. The remaining volunteers are assigned to the placebo group. This is an example of O an observational study. O a completely randomized design. O a matched pairs design. O a block design but not matched pairs.