Answer: D) It is affected by tides.
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Step-by-step explanation:
An estuary is where a river pours into the ocean. So that means fresh water mixes with salt water here. This allows us to rule out choices A and B.
According to NOAA, their site says "Estuaries are one of the most productive ecosystems in the world, so there is a great diversity of animals and plants that live there". In other words, a lot of plants and animals live here, so we can rule out choice C.
Choice D is the answer because tides do affect estuaries. This is simply because the ocean is pretty much right next to it, or attached to it. So it's expected that a high tide would cause the water level in an estuary to increase.