【单选题】

The usual way to ensure reliable delivery is to provide the{{U}} (20) {{/U}}with some feedback about what is happening at the other end of the line. Typically, the protocol calls for the receiver to send back special{{U}} (21) {{/U}}flame bearing positive or negative{{U}} (22) {{/U}}about the incoming frames. If the sender receives a positive acknowledgement about a frame, it knows the frame has arrived safely. On the other hand, a negative acknowledgement means that’ something has gone wrong, and the frame must be transmitted again.
An additional complication comes from the possibility that hardware troubles may cause a flame to{{U}} (23) {{/U}}completely. In this case, the receiver will not react at all, since it has no any reason to react. It should be clear that a protocol in which the sender transmits a frame and then waits for an acknowledgement, positive or negative, will hang forever if a frame is ever lost due to, for example,{{U}} (24) {{/U}}hardware.

A、actingB、workingC、malfunctioning
D、functioning