The Internet is like a complex and diverse virtual world, and the information on each Internet host constitutes a micro virtual world  If the information of the host can be accessed on a host, the host is said to be connected to itself; If the host x can access the information of the host y through the Internet, it is said that x is connected to y
The four Internet hosts with known codes of a, B, C and d have the following information: (1) Host A is connected to any host that is not connected to C; (2) D the host is not connected to C; (3) Host C is connected to any host connected to A
If host D is connected to host B, host B is connected to host A