Answer:
They make many state borders, like the Mississippi river, for example. It forms many borders in the East.
The Columbian Exchange connected the Eastern and Western Hemispheres by using same things like food, livestock, and crops.
Answer: They are called CONTROLS.
Control in an experiment can be defined as an entity that remains the same throughout the experiment. A control in an experiment must remain the same in order to get accurate result. This is done by comparing the results of the experiment with the control.
Controls are used in experiment where some not so obvious factors that might affect the experiment significantly might be present.
