Male amphibians, including frogs and toads, have been plagued by feminization, deformity, behavioral abnormalities and sterility. Biologists from the University of Florida investigated whether reproductive problems among populations of cane toads ( Bufo marinus) are caused by poisons from chemicals associated with agriculture. The biologists collected local adult cane toads from more than 20 different locations. Toads were collected from areas close to agriculture, both large-scale and small-scale farms, as well as from suburbs that are nonagricultural areas. Based on this study, what is the control group