The universe of hypertext servers that allow text, graphics, sound, and other multimedia files to be viewed togather and navigated via hypertext links. It is now the fastest growing area of the Internet. It is

A．when the attenuation on a medium is greater than 25%.
B．when the white noise on the medium exceeds 50% of the medium"s bandwidth
C．when the achievable data rate of the medium exceeds the data rate of the digital signals to be transmitted
D．when the useful bandwidth of the medium exceeds the required bandwidth of the digital signals to be transmited

A．189.132.2.1
B．255.255.255.0
C．198.73.265.50
D．192.0.0.3