Organisms which contain only one cell are?

bacteria, protists, and yeast.

Explanation:Unicellular organisms are made up of only one cell that carries out all of the functions needed by the organism, while multicellular organisms use many different cells to function. Unicellular organisms

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answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.