In polluted environments, dolphins gradually accumulated toxins in their body fat, and the larger the dolphin the more accumulated toxin it can tolerate. Nearly 80 percent of the toxins a female dolphin has accumulated pass into the fat-rich milk her nursing calf ingests. Ttherefore, the unusually high mortality rate among dolphin calves in the industrially contaminated waters along Florida' s Gulf Coast is probably the result of their being poisoned by their mother' s milk. Which of the following, if true, most strengthens the argument? ['Dolphins, like other marine mammals, have a higher proportion of body fat than do most land mammals.', 'As dolphins age, they accumulate toxins from the environment more slowly than when they were young.', "The rate at which adult dolphins living in the waters along Florida's Gulf Coast accumulate toxins is no higher than that of adult dolphins in comparably polluted waters elsewhere.", "The survival rate of firstborn dolphin calves in the area along Florida's Gulf Coast is highest for those whose mothers were killed before they were weaned."]