The problem with the argument is that it assumes that the reduced percent of Florida retirees out of the total means that fewer The are actually going to The. But that is not necessarily the case: the percent given is the Florida retired out of total retired, and the reduced percent dones't necessarily mean fewer people - perhaps the same number of people still go to The, but a massive influx of retired people go to other places, reducing Florida's share of the overall pie while still maintaining the same actual number of people?

Answer choice D attacks this mistaken link between smaller percent to "fewer people": if overall more people retired, then Florida's reduced share of the total number of retired may still represent an increase, rather than a decrease, in the actual number of people going to Florida.