B.
A is wrong because: Planting food and having a steady harvest time and regular schedule for it is more reliable than
C is wrong because: Seeing as people would have been encouraged to stay where they were as to wait for the food to grow, they wouldn't want to move around as much. It makes more sense for them to drop their nomadic lifestyle and settle down on good land to farm.
D is wrong because: People had been trading each other for a long, long time before the development of agriculture. This has nothing to do with it.
