Calculus dilemma - find the assumption

Historian: Leibniz, the 17th century philosopher, published his version of calculus before Newton did. But then Newton revealed his private notebooks, which showed that 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 independently discovered calculus.

Which one of the following is an assumption required by the historian's argument?

(A) Leibniz did not tell anyone about calculus prior to publishing his version of it.
(B) No third person independently discovered calculus prior to Newton and Leibniz.
(C) Newton beleived that Leibniz was able to learn something important about calculus from his letter to him.
(D) Neither Newton nor Leibniz knew that the other had developed a version of calculus prior to Leibniz's publication.
(E) Neither Newton nor Leibniz learned crucial details about calculus from some third source.

The author says that Newton and Leibniz independently discovered calculus because the letter of newton to leibniz did not reveal anything major about calculus. So, we need to find that statement which shows that no other factor was involved in both of them getting to know about calculus. Only then can this argument be valid.

A) This is not relevant to the argument and not an assumption
B) This is a fact, not an assumption
C) Not an assumption, if true, this weakens the conclusion that they both discovered it independently.
D) This is incorrect because Newton had shared a letter with Leibniz about calculus.
E) This is the assumption which, if true, strengthens the argument of the author. If both of them had not learnt any details from a third source, then it helps strengthens the conclusion of the author.

Hence, E!