Ringtail opossums are an Australian wildlife species that is potentially endangered. A number of ringtail opossums that had been orphaned and subsequently raised in captivity were monitored after being returned to the wild. Seventy-five percent of those opossums were killed by foxes, a species not native to Australia. Conservationists concluded that the native ringtail opossum population was endangered not by a scarcity of food, as had been previously thought, but by non-native predator species against which the opossum had not developed natural defenses. Which one of the following, if true, most strongly supports the conservationists' argument? ['Ringtail opossums that grow to adulthood in the wild defend themselves against foxes no more successfully than do ringtail opossums raised in captivity.', 'Foxes, which were introduced into Australia over 200 years ago, adapted to the Australian climate less successfully than did some other foreign species.', 'The ringtail opossums that were raised in captivity were fed a diet similar to that which ringtail opossums typically eat in the wild.', 'There are fewer non-native predator species that prey on the ringtail opossum than there are native species that prey on the ringtail opossum.']