Among the choices posted, the best and only sensible answer is: the search for food items. The Asians who crossed the Bering Strait did not need to spread religion, to bring slaves from Africa nor the desire to conquer new peoples.
They were hindered by economic development and lack of enforcement, so I'd say 1. Cost, because no one would pay for it.
The life story of the Buddha begins in Lumbini, near the border of Nepal and India, about 2,600 years ago, where the man Siddharta Gautama was born.
Although born a prince, he realized that conditioned experiences could not provide lasting happiness or protection from suffering. After a long spiritual search he went into deep meditation, where he realized the nature of mind. He achieved the state of unconditional and lasting happiness: the state of enlightenment, of buddhahood. This state of mind is free from disturbing emotions and expresses itself through fearlessness, joy and active compassion. For the rest of his life, the Buddha taught anyone who asked how they could reach the same state.
“I teach because you and all beings want to have happiness and want to avoid suffering. I teach the way things are.”
– The Buddha
