Economic systems
natural systems.
a. operate within
b. are independent of
c. have no relation to
d. are equal to

Your answer would be :

hope it helps!

primary succession begins with a bare rock and ends with a forest secondary succession begins after a natural disaster where there is a temporary setback after a forest fire or local destruction, but the surrounding area can fill in

the relative standard reduction potential of the half-cell in which reduction occurs; more positive than the other half-cell. reduction is a chemical process in which electrons are added to an atom or an ion; it always occurs accompanied by oxidation of the reducing agent.