Passage A tells us on lines 8-10 that "now...ecologists realize that when humans introduce new species..even pristine, i.e. undisturbed, habitats are threatened" in contrast to Elton's view that disturbed habitats are most vulnerable. Passage B does not discuss Elton's view or whether it is shared by the ecologists.
Let's look at the rest of the answer choices. (A) Neither passage talks about the economic impacts of introduced species. (B) Neither passage says that ecologists use inconsistent criteria. (C) Both passages mention that invasive species may lead to extinction but neither passage asserts that most ecologists agree that to be the cast. (D) Neither passage discusses ecologists' views before Elton.