(D) "If each step in a series of changes involves a decrease of one attribute of the thing undergoing the change and an increase of another, the series will terminate with the first attribute eliminated and only the second attribute present."
This mirrors the logic in the passage and is correct. The steps in a series of changes refers to North America's shift from wood to coal and then from coal to oil and natural gas, wherein carbon is decreased and hydrogen is increased with each shift. Therefore, the passage concludes that the first attribute- carbon- will be eliminated and only the second attribute- hydrogen- is left to be the only thing present.
(E) "If one substance is better for a certain purpose than another substance is, then the best substance for that purpose is one that includes among its attributes all of the attributes of the first substance and none of the attributes of the second substance."
This is incorrect and a worse answer than (D) because it does not track the passage's logic as clearly. The passage never discusses any substance as better or worse than another, nor does it specify a certain purpose. Moreover, the passage does not discuss the attributes that make up carbon and hydrogen. So, (D) expresses a general principle that underlies the passage better than (E) does.
Does that make sense? Please let us know if you have any other questions!
