A local chemical plant produces pesticides that can cause sterility in small mammals such as otters. Soon after the plant began operating, the incidence of sterility among the otters that swim in a nearby river increased dramatically. Ttherefore, pesticides are definitely contaminating the river. Which one of the following arguments contains a flaw in reasoning that is similar to one in the argument above? ['A diet low in calcium can cause a drop in egg production in poultry. When chickens on a local farm were let out in the spring to forage for food, their egg production dropped noticeably. So the food found and eaten by the chickens is undeniably low in calcium.', 'The bacteria that cause tetanus live in the digestive tract of horses. Tetanus is a highly infectious disease. Consequently it must be that horses contract tetanus more frequently than do most other animals.', 'Animals that are undernourished are very susceptible to infection. Animals in the largest metropolitan zoos are not undernourished, so they surely must not be very susceptible to disease.', 'The only animal that could have produced a track similar to this one is a bear. But there are no bears in this area of the country, so this animal track is a fake.']