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) flame bearing positive or negative (73) 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 flame 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.

（71）处应填（）。

A．receiver

B．controller

C．sender

D．customer

下图中，甲图是菜豆种子结构图，乙图是玉米种子结构图，丙图是菜豆幼苗图，请据图回答。

（1）在菜豆种子中，[5]     是新植物体的幼体，它是由菜豆花结构中的     发育而来的。

（2）根据所学知识，判断并识别丙图中菜豆幼苗A部分是由菜豆种子结构中的[2]    发育而来的。

（3）玉米种子萌发时必须要有足够的营养供应，萌发时所需的营养物质来自于[        ]胚乳中。