Flowing water eroding a limestone riverbed

a
Chemical Change
b
Physical Change

they are both synthetic elements

explanation:

d

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.

b) nuclear ive learned that in a pedtrodic table