In paragraph 2, why does the author talk about what happened as a result of North and South America becoming joined at the Isthmus of Panama? A. To make the point that predation can have as much effect on species survival as competition does B. To show how the ability to switch to an alternative resource can give a species a competitive advantage C. To account for the current species composition of North and South America D. To provide an example of the serious effects of competition between unrelated species