In an arbitrary group of six people, which of the following situations is impossible?

• A. At least five of them known each other and two of them do not know each other.
• B. At least four of them known each other and three of them do not know each other.
• C. No three of them know each other and no three of them unknown to each other.
• D. No four of them are unknown to each other and there is a person known to every other.