Which is not a characteristic of minerals
a. are solids with a fixed shape and volume
b. are organic in nature
c. are naturally occuring
d. have a definite chemical position
e. have a definite crystal structure

I think d is wrong because  Everything else is correct for sure. also, minerals vary in the position they hold and have different chemical compounds.

your answer would be a. 12.5 kg

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.