In the past century, North America has shifted its main energy source first from wood to coal, then from coal to oil and natural gas. With each transition, the newly dominant fuel has had less carbon and more hydrogen than its predecessor had. It is logical to conclude that in the future the main energy source will be pure hydrogen. Which one of the following expresses a general principle that could underlie the argument? ['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.', 'If the second stage of a process has been completed more quickly than the first stage, the third stage of that process will be completed more quickly than the second stage.', 'If each of two desirable attributes belongs to a useful substance, then the most useful form of that substance will have those two attributes in equal amounts.', '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.']