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.