Dolphins are mammals and sharks are fish. They both have fins. This suggests that . dolphins and sharks inherited fins from a recent common ancestor
both dolphin and shark fins are vestigial features
a DNA mutation caused some sharks to evolve into dolphins
dolphin and shark fins are analogous structures

chemistry sucks a**

explanation:

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.