Getting these when you’re young your immune system fight off diseases:

a. bandages

b. stickers

c. shots (immunizations)

d. cough drops

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

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.