Ais any stimulus that increases the probability that a preceding behavior will occur again. catalyst rejoinder reinforcer stimulant

A reinforcer is a stimulus that increases the probability that the behavior will occur again.  An example of a reinforcer would be a dog treat.  If you want your dog to follow the command "sit", you would follow the behavior with a dog treat, or reinforcer, to ensure that the behavior of "sit" would occur again.

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.