IP routing protocols， such as OSPF and Enhanced EIGRP， are used primarily in network designs that contain entry and exit points to the network from service providers. What is a common trait of IP routing  protocols？（）  

A. They add needed overhead in smaller deployments.
B. They are essential for optimum and redundant forwarding.
C. They are used primarily in larger network designs，to support different networks and single entry and  exit points.
D. They are usually necessary in smaller deployments.