On a typical position-versus-time graph, time is a. positive b. negative c. positive or negative

it is A positive cause u cant go negative position and time

hope this helped ^_^

i think it's c. i'm not sure but i think so.

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.