What is the best way to ensure that results of a study are generalizable to a population?

In 1914, alabama produced 1.7 million bales of cotton, but three years later produced only 515,000, bales. meanwhile, in georgia, 5.2 million acres of land was dedicated for cotton production in 1914, but by 1923 there were just 2.6 million acres used for growing cotton. all of this was the result of the work of a tiny insect: the boll weevil. alabama and georgia first felt the effects of the boll weevil at different times. what would account for this difference in timing? a) the infestation moved from west to east. b) the infestation moved from east to west. c) georgia had better ways to deal with the problem. d) alabama was further from the insect's point of origin.