How is the body a system of interacting subsystems.

Abody contains many subsystems within it. for instance, it consists of the circulatory system which has its own channels and vessels for the blood to flow. this system has its own pump for circulating the blood throughout the body and regulates on its own. similarly, the body has digestive system which stretches from oesophagus right down to rectum. it also consists of its own channels for passage of food, the right set of muscles present in walls of intestines to move food along and a connection to an external excretory pathway. hence, body appears as one system but has many sub systems operating within it. 

Covalent bond is the correct answer