“人生道路虽然漫长，但紧要处常常只有几步，特别是当人年轻的时候，有些岔道口，你走错一步，可以影响人生的一个时期，也可以影响一生。”这句话告诉我们

①青春时节，是事业的辉煌时期 

②青春时节，是人生的关键时期 

③我们应该迈好青春第一步 

④人生路上一点错误也不能犯

A．①②③

B．②③④

C．②③

D．①③

The usual way to ensure reliable delivery is to provide the (16) 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 (17) frame bearing positive or negative (18) 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 frame to (19) 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, (20) hardware.

（20）是（）

A.acting

B.working

C.malfunctioning

D.functioning