A museum wants to store their valuable documents in cases that contain a gas that will protect the documents. The museum director should choose Choose... = chlorine gas, oxygen gas, a noble gas, a halogen.
She should not choose one of the other gases because they are too
Choose... = expensive, reactive, smelly, unreactive.

the answer is definitely c.

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.