单选题 选出应填入下面一段英语中______内的正确答案。
   TCP functions at the Open System Interconnection (OSl) (1) layer, or Layer 4. Its chief responsibility is to ensure (2) end-to-end connectivity. IP, located one layer down, at the OSI network layer, or Layer 3,communicates the addresses of each (3) sender and receiver to the routers along the way. Routers and Layer 3 switches can read IP and other Layer 3 (4) . This information, combined with routing tables and other network intelligence, is all it takes to get (5) the room or around the world via TCP/IP.