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） hardware.。

A．vanish

B．vary

C．appear

D．incline

烃类原料在裂解炉内进行高温裂解，产出组成为氢气、低/高碳烃类、芳烃类及馏分在（）摄氏度以上的裂解燃料油的裂解气混合物。

A、288

B、388

C、488

D、588