The goblin fern, which requires a thick layer of leaf litter on the forest floor, is disappearing from North American forests. In spots where it has recently vanished, the leaf litter is unusually thin and, unlike those places where this fern still thrives, is teeming with the European earthworm Lumbricus rubellus, which eats leaf litter. L. rubellus is thus probably responsible for the fern' s disappearance. Which one of the following is an assumption on which the argument depends? ['None of the earthworms that are native to North America eat leaf litter.', 'Wherever there is a thick layer of leaf litter in North American forests, goblin ferns can be found.', 'L. rubellus does not favor habitats where the leaf litter layer is considerably thinner than what is required by goblin ferns.', 'Dead leaves from goblin ferns make up the greater part of the layer of leaf litter on the forest floors where the goblin fern has recently vanished.']