A fourteen-year study of finches on the Galapagos islands concluded that there is a definite relationship between climate and the population size of finch species that thrive at various times. During droughts, more members of large finch species survive because their bills are large enough to crack large, hard seeds, giving them a food supply unavailable to smaller birds. In rainy years, fewer members of the large finch species survive because the additional moisture fosters the growth of plants that produce small seeds. The larger finch varieties have to consume enormous numbers of small seeds to meet their energy demands, and some just cannot eat them fast enough. Which one of the following must be assumed in order to justify the conclusion that climatic variations cause a major difference in survival rates of small and large finches? ['During drought conditions, the weather promotes the growth of plants that produce small, hard seeds.', 'A lengthy period of rainy weather results in fewer large, hard seeds being produced.', 'In rainy periods, the small finches gather enough food to grow much larger and heavier, but their ultimate size is limited by their inability to eat small seeds fast.', 'The Galapagos climate during this fourteen- year period had about as much dry weather as it had wet weather.']