Organisms change their behavior in responses to changes in the environment.
All living things can respond to stimuli in the exterior environment, it is a form of energy-light waves or sound vibrations, From what they sense they will make necessary changes to their diet and or where they are located. Such as winter weather which is a type stimuli will cause some animals to start talking.
Answer: 2. cannot be an element.
Explanation:
Element is a pure substance which is composed of atoms of similar elements.It can not be decomposed into simpler constituents using chemical reactions.Example: Copper 
Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass.It can be decomposed into simpler constituents using chemical reactions. Example: methane 

Thus as it can be decomposed into its constituent elements, it is a compound.
Mixture is a substance which has two or more components which do not combine chemically and do not have any fixed ratio in which they are present.
Thus methane cannot be an element.
Different amount of species in an area.
The skeletal muscle ph is typically 7.15