Which of the following, if true about Plazos but not about nearby islands, contributes most to an explanation of the long-standing tendency of iguanas on Plazos to be smaller than those of the same age on nearby islands?

　　Periodic wind shifts cause extended dry spells on Plazos every year, putting the larger iguanas, whose bodies require relatively more water, at a great disadvantage.

　　There are exactly three species of iguanas on Plazos but only two species of seagulls that feed on iguanas, and a relatively small percentage of each year s hatchlings are consumed by seagulls.

　　Wild cats, which were introduced as pets by early settlers and which were formerly major predators of Plazos iguanas, were recently killed off by a disease specific to cats.

　　The iguanas on Plazos are a relatively ancient part of the island s animal life.

　　Both land and marine iguanas live on Plazos,and the land iguanas tend to be larger than marine iguanas of the same age.

　　7. Every human being who has ever lived had two parents. Therefore, more people were alive three thousand years ago than are alive now.The reasoning in the argument is flawed because it

　　overlooks the number of people in each generation during the last three thousand years who left no descendants

　　disregards possible effects of disasters such as famines and plagues on human history

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