In a national park located on an island, a herd of moose was increasing in number and threatening to destroy species of native plants. Wolves were introduced to the island to reduce the herd and thereby prevent destruction of the vegetation. Although the wolves prospered, the moose herd continued to grow. Which one of the following, if true, most helps to explain the failure of the strategy involving wolves? ['The presence of wolves in an area tends to discourage other predators from moving into the area.', 'Healthy moose generally consume more vegetation than do those that are diseased or injured.', 'Attempts to control moose populations in other national parks by introducing predators have also been unsuccessful.', 'Wolves often kill moose weakened by diseases that probably would have spread to other moose.']