One important impact of the Mongol expansion across Asia and Europe was the "<span>(2) rise in trade along the Silk Roads" but it should be noted there were others as well.</span>
The best description of Nirvana as it applies to the Buddhists would be that it is the ultimate goal--one in which the human is free from all earthy desires and has obtained true enlightenment.
A. Climate conditions and C. Available food
they claimed that without slaves the economy would collapse I think, I'm not quite sure if I'm remembering correctly though