Trades.
For example; the trade of wheat, other cash crops and natural resources
Answer:
Hand washing has external health benefits, helping prevent the spread of communicable diseases. If a program were somehow devised so that people received a small reward every time they washed their hands, we would expect the frequency of hand washing to increase.
Explanation: People who are given something (A reward of some sort) are proven to do something more, (in this case washing their hands more), or better then they would do without a source of motivation.
