Which of the following two cycles are the most closely related in their role in sustaining life?
a.
carbon and nitrogen
b.
water and nitrogen
c.
carbon and oxygen
d.
nitrogen and phosphorus

C: carbon and oxygen cycles are the most closely related in their role in sustaining life because they both include processes of exchanging themselves among different parts of the earth. E.g. they both process through the ground, in the air, etc.