What historical factors prompt yali to ask his question about cargo

Yali asks: Why is it that the whites developed so much cargo ... but we blacks had so little cargo ?. There are 4 factors to consider: 1. Availability of wild plants and animals for domestication. 2. Barriers to diffusion and migration within a continent. 3. Barriers to diffusion and migration between continents. 4. Size and density of the population

i'm pretty sure i had this question the other day and b was the right answer.