When feeding, aquatic birds known as phalaropes often spin rapidly on the water' s surface, pecking for food during each revolution. To execute these spins, phalaropes kick one leg harder than the other. This action creates upwelling currents. Because plankton on which phalaropes feed typically occurs in greater quantities well below the surface, it is hypothesized that by spinning phalaropes gain access to food that would otherwise be beyond their reach. Which of the following, if true, most strongly supports the hypothesis? ['Phalaropes rarely feed while on land.', 'Phalaropes do not usually spin when food is abundant at the surface.', 'Phalaropes sometimes spin when they are not feeding.', 'A given phalarope spins exclusively either to the right or to the left.']