In paragraph 3, the author contrasts a variety of organisms to illustrate which of the following points? A. Geographic barriers are less likely to keep apart populations of plants than populations of animals. B. Geographic barriers are more likely to keep apart populations of large organisms than populations of small organisms. C. Some members of a species are able to cross geographic barriers, while other members of the same species are not. D. The effectiveness of geographic barriers in keeping organisms apart depends on an organism's ability to move across barriers.