Each type of ecosystem works best with the amount of rainfall it usually gets. Sometimes an ecosystem goes through environmental changes like floods or droughts.

How might a flood or a drought affect plants and animals living in a particular ecosystem?

It could cause some animals to move to new areas.
It could make life easier for some plants or animals.
It could cause some plants and animals to die.

An ecosystem is a system within systems such as soil, plant, and animals systems. These systems are influenced by the level of energy they interact with.  

Hence the option A is correct.

Learn more about the works best with the amount of rainfall it usually gets.

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It could cause some plants and animals to die.

Explanation:

The reason I picked this answer is because if there is a drought then nonw of the plants or animals have enough water to survive which in the end kills then. But if there is to much water such as a flood then it will most deffinitly kill many plants wether that be from the strong flow of the water uprooting the plant or from it being over watered. And the amount of water will be deadly for other animals.

these civilizations were located around fertile rivers, where they could produce an abundance of crops