They are called bellwether species, also known as sentinel species.
Answer:
The Great Depression
Explanation:
Pretty self-explanatory. The Dust Bowl did further increase the damage caused by the great depression but that happened midway.
The answer is: c. his mother gave him his allowance for doing his chores
Secondary reinforcement refers to something that could influence other people to do a certain behavior that come from other thing beside their own will.
When a mother gives allowance for his chores, it could make the children much more likely to do the chores. But the increase in likelihood does not come from the children's will, but from the children's intent to get the money,.
