Beck made the following table to compare prokaryotic and eukaryotic cells. Structure Prokaryotic cell Eukaryotic cells Cell wall is present Yes Sometimes Flagellum for movement Yes No X is present No Yes Which of the following would best replace X? DNA
Cell membrane
Nucleus
Cytoplasm

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.