'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 best summarises the subject of this paragraph?