18. What is the relationship between the respiratory and circulatory systems?
A The respiratory system brings in oxygen for the circulatory system to pump around the body.
B. The respiratory system brings in oxygen for the digestive system to pump around the body.
C. The digestive system brings in oxygen for the circulatory system to pump around the body.
D. The circulatory system brings in oxygen for the respiratory system to pump around the
body.​