Historian: Leibniz, the seventeenth-century philosopher, published his version of calculus before Newton did. But then Newton revealed his private notebooks, which showed he had been using these ideas for at least a decade before Leibniz' s publication. Newton also claimed that he had disclosed these ideas to Leibniz in a letter shortly before Leibniz' s publication. Yet close examination of the letter shows that Newton' s few cryptic remarks did not reveal anything important about calculus. Thus, Leibniz and Newton each independently discovered calculus. Which one of the following is an assumption required by the historian's argument? ['Leibniz did not tell anyone about calculus prior to publishing his version of it.', 'Neither Newton nor Leibniz learned crucial details about calculus from some third source.', 'No third person independently discovered calculus prior to Newton and Leibniz.', "Neither Newton or Leibniz knew that the other had developed a version of calculus prior to Leibniz's publication."]