Wide Area Network (WAN) is a computer network that covers a broad areA. Which statement is correct for wide-area network (WAN)?（）

• A、Connects client devices wireless protocol
• B、Crosses metropolitan, regional, or national boundaries
• C、Limited in a single building, a group of buildings, or even a single room
• D、Encompasses an entire city or metropolitan area and connects multiple local-area networks (LANs)