The correct answer is C) The Mongolians allied with Japan so that China would have to fight on two fronts.
The statement that is NOT a reason why the Mongolians were able to conquer such a vast area of land is "The Mongolians allied with Japan so that China would have to fight on two fronts."
What is true abut the Mongolian Empire was that it had a powerful army and a cavalry full of fierce soldiers very committed to the cause of the emperor. That is why they were capable of conquering vast lands and increase the dominion of Mongolian expansion. Another important reason was that they had good strategies and tactics that gave them an advantage when waging war.
