At a certain period in Earth' s history, its atmosphere contained almost no oxygen, although plants were producing vast quantities of oxygen. As a way of reconciling these two facts, scientists have hypothesized that nearly all of the oxygen being produced was taken up by iron on Earth' s surface. Clearly, however, this explanation is inadequate. New studies show that the amount of iron on Earth' s surface was not sufficient to absorb anywhere near as much oxygen as was being produced. Ttherefore, something in addition to the iron on Earth' s surface must have absorbed much of the oxygen produced by plant life. In the argument given, the two portions in boldface play which of the following roles? ['The first is a judgment made by the argument about a certain explanation; the second is that explanation.', "The first sums up the argument's position with regard to a certain hypothesis; the second provides grounds for that position.", 'The first is a concession by the argument that its initial formulation of the position it seeks to establish requires modification; the second presents that position in a modified form.', "The first expresses the argument's dismissal of an objection to the position it seeks to establish; the second is that position."]