Australia has considerably fewer species of carnivorous mammals than any other continent does but about as many carnivorous reptile species as other continents do. This is probably a consequence of the unusual sparseness of Australia' s ecosystems. To survive, carnivorous mammals must eat much more than carnivorous reptiles need to; thus carnivorous mammals are at a disadvantage in ecosystems in which there is relatively little food. Which one of the following most accurately expresses the main conclusion of the argument? ['The reason that carnivorous mammals are at a disadvantage in ecosystems in which there is relatively little food is that they must eat much more in order to survive than carnivorous reptiles need to.', "The unusual sparseness of Australia's ecosystems is probably the reason Australia has considerably fewer carnivorous mammal species than other continents do but about as many carnivorous reptile species.", "Because Australia's ecosystems are unusually sparse, carnivorous mammals there are at a disadvantage relative to carnivorous reptiles.", 'In ecosystems in which there is relatively little food carnivorous mammals are at a disadvantage relative to carnivorous reptiles.']