Answer:
D. the respiratory and circulatory systems
Explanation:
The human respiratory and circulatory systems work together to supply the body with oxygen and get rid of waste carbon dioxide. While the former deals with air and the latter with blood, they work together seamlessly by coordinating the functions of the many parts of each system. Other systems in the body, such as the digestive system and the nervous system, are important too, but the circulatory and respiratory systems have to work continuously, usually without pausing even for a few minutes.
