How many moles of carbon are in 7.87x10^7 carbon atoms?

the answer to your question is c.

if you read the house of usher you should know that the house was haunted

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.