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问题 单项选择题

当光源移到无穷远处时，()所得的图像就是这个物体的平行投影。

(A)投影线相交于一点;

(B)投影线互相平行;

(C)投影线杂乱无章;

(D)没有投影线

答案

参考答案：B

单项选择题

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.

（72）处应填（）。

A．data

B．control

C．request

D．session
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填空题

已知△ABC中∠ACB=90°，CD⊥AB于D，∠A=30°，BC=2cm，则AD=_______．
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