Active muscles demand huge amounts of oxygen, and the circulatory system works hard to provide it. Blood carries oxygen to the muscles. When the body is at rest, blood carries nutrients to the muscles so that they can repair and rebuild themselves. Blood also carries away the waste products that hard-working muscles produce, back out through the lungs to be filtered.
so they Circulatory system xx muscular system :)
Answer: C) a factor that may affect an experiment or investigation
Explanation:
This is the best answer. A and B can be eliminated. D describes a controlled variable which is more specific than a scientific variable, thus C is the best answer. It is the best description of just a scientific variable.
D. Phenotype because a phenotype is observable traits