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问题 填空题

装满氧气的集气瓶，按如图所示，用带火星的木条分别以甲、乙两种方式迅速插入，发现木条复燃，且在甲中燃烧比在乙中更旺。上述实验说明了氧气具有的性质是：

①                                                  ②                                                 

答案

①助燃性；②氧气的密度比空气的密度大

选择题

2010年世界博览会将在中国     举行，也是历来首次由中国举办的世界博览会。这次世博会的主题是“城市，让生活更美好”（Better City, Better Life）。

A．上海

B．南京

C．北京

D．杭州
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单项选择题

The usual way to ensure reliable delivery is to provide the (16) 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 (17) frame bearing positive or negative (18) 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 frame to (19) 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, (20) hardware.

（20）是（）

A.acting

B.working

C.malfunctioning

D.functioning
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