One of these pioneer ecologists was Frederick Clements, who studied ecology extensively th

roughout the Midwest and other areas in North America. He held that within any given region of climate, ecological communities tended to slowly converge toward a single endpoint, which he called the "climatic climax." This "climax" community was, in Clementss mind, the most well-balanced, integrated grouping of species that could occur within that particular region. Clements even thought that the process of ecological succession—the replacement of some species by others over time—was somewhat akin to the development of an organism, from embryo to adult. Clements thought that succession represented discrete stages in the development of the community(rather like infancy, childhood, and adolescence), terminating in the climatic "adult" stage, when the community became self-reproducing and succession ceased. Clementss view of the ecological community reflected the notion of a precise balance of nature.

In paragraph 5, why does the author mention green ash trees and plains cottonwood trees？

A．To support the current view about how ecological communities develop

B．To provide an example of species that prefer to live on floodplains

C．To provide evidence that supports the theory of Clements

D．To show where one ecological community stops and the other begins