Body systems work together to maintain which type of constant environment?

A internal.
B external. C rigid. D abdominal

Option A is correct. (Internal).

Explanation:

The body systems work together to maintain the constant internal environment in response to external stimuli in a process called homeostasis.

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.