PLZ SOMEONE HELP THANKS:
materials and conclude which is best to use. Full explanation

For quick facts use reference materials such as encyclopedias, almanacs, or dictionaries. For credible and reputable information, use online databases of academic journals, or peer-reviewed journal articles that are written by scholars and researchers for a specific discipline.

a compound is a molecule made of atoms from different elements.

a. by the combination of an unlimited number of elements

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.