What Two Types of Organisms Tend to Be Good Indicator

What two types of organisms tend to be good indicator species, and why?

A) Anything works, scientists just have to agree on it.
B) Whatever is most rare in the ecosystem in question
C) Large mammals because they're easy to find
D) Amphibians, because if they are there, there are sufficient wet and dry areas
E) Birds and flowering plants, because they are easy to survey.