Which statement describes the best procedure to determine if a vaccine for a disease in a certain bird species is effective?
(a) vaccinate 100 birds and expose all 100 to the disease.
(b) vaccinate 100 birds and expose only 50 of them to the disease.
(c) vaccinate 50 birds, do not vaccinate 50 other birds, and expose all 100 to the disease.
(d) vaccinate 50 birds, do not vaccinate 50 other birds, and expose only the vaccinated birds to the disease.