The usual way to ensure reliable delivery is to provide the (71) 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 (72) frame bearing positive or negative (73) about the incoming frames. If the sender receives a positive acknowledgement about a frame, it knows the 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 frame to (74) 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, (75) hardwareo

A．application

B．connection

C．stream

D．acknowledgement

氧气呼吸器坚持每年拆装检修，连续（）以上未使用的氧气呼吸器，清净罐内的药剂必须更换。

A、一个月

B、两个月

C、三个月

D、四个月