`Religions, like camel caravans, seem to avoid mountain passes. Buddhism spread quickly south from Buddha's birth-place in southern Nepal across the flat Gangetic plain to Sri Lanka. But it took a millennium to reach China...The religious belt stretched eventually to Mongolia and Japan, but in Afghanistan Buddhism filled only a narrow belt that left pagans among the valleys to the east and west in Kailash and Ghor'
Which of the following, if true, would most weaken the conclusion to the above argument?