Which statement best describes ecosystems? A. they can never change once they are established
B. they constantly change in different ways over time
C. ecosystems always revert back to their original state
D. each ecosystem can exist for only a specific lenght of time

b. they constantly change in different ways over time.

Explanation:

different factors affect ecosystems which end up changing them in different ways

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.