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．application

B．connection

C．stream

D．acknowledgement

肌沟法臂丛阻滞的缺点有哪些（）

A.尺神经阻滞不全或较迟

B.有损伤椎动脉的可能

C.有误入蛛网膜下隙和硬膜外下腔的危险

D.低位法可产生刺破胸膜产生气胸

E.不宜同时进行两侧阻滞