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Redundancy and fault tolerance


A group of organizers are putting together a conference called DevFest and want to make sure the Internet connection will be reliable. They partner with two Internet providers, Komkastic and SupaSonic.
This diagram illustrates their connection to the Internet:
Diagram of network. A series of laptop computers are shown on left, labeled DevFest. Two wireless signals emanate towards those laptops from two routers, labeled Komkastic and SupaSonic. The routers are connected to a network with four other routers labeled A, B, C, and D.
  • Line connects Komkastic's router to Router D.
  • Line connects SupaSonic's router to Router D.
  • Line connects Router D to Router A.
  • Line connects Router D to Router B.
  • Line connects Router D to Router C.
Routers A, B, and C have 3 arrows each coming out from them, indicating a continuing network.
Each circle is a router, each line is a connection, and the lines with arrows indicate connections to further routers on the Internet.
Unfortunately, this network is not fault tolerant: there's still a single point of failure.
Which router needs to stay available for DevFest to have a working connection to the Internet?
Choose 1 answer: