Their marginal
people keep spending additional units of a particular resource on a want until their marginal benefit is
cost.
decreasing with
greater than
less than
not affected by

The correct answer is a. when they started making tools that will them in cultivating lands, they could raise crops and that made them attached to one place for a longer period of time. after that, they started domesticating animals and building their own permanent settlements that resemble the ones we have today.