How did geologists decide where one division of the geologic time scale ends and the next begins

What factors influenced brazil's current and future economic strength?

If u make mistakes on a school trip to knotts like standing up on a ride too early and they ask you not to do it again or and if you do it again you will be escorted out will it go on my student record? yes i know it is a dumb question

Ascientist makes a map using gis. one layer is an aerial photograph of farmland. other layers show water level changes and how crop cover has changed over time. what might the scientist be studying?