In paragraph 4, what is the author's main purpose in the discussion of the dramatic cessation of tree growth at the upper timberline? A. To argue that none of several environment factors that are believed to contribute to that phenomenon do in fact play a role in causing it. B. To argue in support of one particular explanation of that phenomenon against several competing explanations C. To explain why the primary environmental factor responsible for that phenomenon has not yet been identified D. To present several environmental factors that may contribute to a satisfactory explanation of that phenomenon