In information theory, "information" is regarded as knowledge which reduces or removes (71) about the occurrence of a specific event from a given set of possible events. The concept "event" is to be understood as used in the theory of probability. For instance, an event may be the occurrence of a (72) character or word in a given message or in a given position of a message.
In communication theory, channel is defined as that part of a communication system that connects the message source with the message sink. An encoder may be inserted between the message source and the input to the channel, and a (73) between the output of the channel and the message sink. Generally, these two units are not considered as being parts of the channel. In certain cases, however, they may be considered as pans of the message source and message sink, (74) . In information theory (75) to Shannon, the channel can be characterized by the set of conditional probabilities of occurrence of all the messages received at the message sink when a given message emanates from the message source.
（73）处填（）。
A．debugger
B．decoder
C．decompiler
D．demodulator