单选题 .  ① There have been numerous well-documented extinctions of indigenous species caused by the introduction of non-indigenous predators and pathogens. ②However, surprisingly few extinctions of indigenous species can be attributed to competition from introduced species. ③For example, during the past 400 years, 4,000 plant species have been introduced into North America, and these non-indigenous plants currently account for nearly 20 percent of North America's plant species. ④Yet that no evidence exists that any indigenous North American plant species became extinct as a result of competition from new species could mean that such extinctions take longer to occur than scientists initially believed or, alternatively, that extinctions are rarely caused by competition from non-indigenous species.28.  The passage is concerned primarily with ______