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．acting

B．working

C．malfunctioning

D．functioning

患儿，男，12岁，站立不稳和行走困难3年，双手取物时震颤1年。查体：神清，言语缓慢，眼球运动不受限，四肢肌力5级，肌张力正常，腱反射消失，双病理征阳性。患儿站立时足距增宽，Romberg征阳性，双手有动作性和意向性震颤。

若脊髓MRI显示脊髓变细，肌电图提示感觉传导速度减慢，心电图发现心事肥厚、心律失常，考虑哪种病可能性大（）

A.脊髓小脑性共济失调

B.遗传性痉挛性截瘫

C.腓骨肌萎缩症

D.脊肌萎缩症

E.Friedreich型共济失调