Dolphins can swim at high speeds and achieve high acceleration in the water. In 1936, Sir James Gray calculated the force dolphins should be able to exert based on their physiology. <b> He concluded that the propulsive force they were able to exert was not enough to explain how fast they swim and accelerate. </b> In the 2000s, experimenters used special computer-enhanced measurements of the water in which dolphins were swimming. Through mathematical modeling, they were able to measure the force dolphins exert with their tails. As it turns out, dolphins exert considerably more force with their tails than Sir James Gray or anybody else ever expected. <b> Ttherefore, the force exerted by their tails easily explains how fast they swim and accelerate. </b> In the argument, the two portions in boldface play which of the following roles? ['The first is a generally held assumption; the second is a conclusion that violates that assumption.', 'This first is a piece of evidence supporting the main conclusion; the second is the main conclusion.', 'The first is an opinion the author seeks to refute; the second is the opinion the author supports.', 'The first is a false conclusion based on an incorrect premise; the second is the revised conclusion drawn from the corrected premise.']