A scientist made three observations: (1) in the world' s temperate zones, food is more plentiful in the ocean than it is in fresh water; (2) migratory fish in temperate zones generally mature in the ocean and spawn in fresh water; and (3) migratory fish need much nourishment as they mature but little or none during the part of their lives when they spawn. On the basis of those observations, the scientist formulated the hypothesis that food availability is a determining factor in the migration of migratory fish. Subsequently the scientist learned that in the tropics migratory fish generally mature in fresh water and spawn in the ocean. Which one of the following would it be most helpful to know in order to judge whether what the scientist subsequently learned calls into question the hypothesis? ['whether in the tropics food is less plentiful in the ocean than in fresh water', 'whether any species of fish with populations in temperate zones also have populations that live in the tropics', "whether in the world's temperate zones, the temperatures of bodies of fresh water tend to be lower than those of the regions of the oceans into which they flow", 'whether there are more species of migratory fish in the tropics than there are in temperate zones']