Answer:
They make many state borders, like the Mississippi river, for example. It forms many borders in the East.
The correct answer is a primary source
These are sources like letters or photographs or anything similar that directly shows how something happened, without a person analyzing it. A book about those letters would be a secondary source, and a book about that book would be a tertiary one, and so on.
Strengthening the possibility of repeated behavior as a result of rewards is called the positive reinforcement. It involves adding a stimulus to certain situation because you saw that something is done and you want that this action would be repeated. This usually happens during a favorable event or outcome where you encourage or strengthen that event.