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问题 单项选择题

成人心脏按压最佳频率是

A．40～50/min

B．60～70/min

C．80～100/min

D．110～130/min

E．140/min

答案

参考答案：C

解析：按压频率应与正常心率相同。

选择题

为了探究某一非生物因素对植物生长的影响，小明选择了生长状况相仿的三株

幼苗，观察、记录，10天后结果如下表：

环境条件生长结果
1号光照充足，适量浇水叶片颜色翠绿，生长良好
2号光照充足，不浇水叶片萎缩，生长不良
3号光照充足，大量浇水根部缺氧，生长不良
从中可看出探究的非生物因素和对照组分别是

A．水分、2号     B．水分、1号     C．阳光、3号     D．阳光、1号
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单项选择题

A transport layer protocol usually has several responsibilties. One is to create a process-to-process communication UDP uses （1）numbers to accomplish this. Another responsibility is to provide control mechanisms at the transport level UDP does this task at a very minimal level. There is no flow control mechanism and there is no （2）for received packet. UDP, however, does provide error control to some extent. If UDP detects an error in the received packet, it will silently drop it.

The transport layer also provides a connection mechanism for the processes. The （3）must be able to send streams of data to the transport layer. It is the responsibility of the transport layer at（4）station to make the connection with the receiver chop the stream into transportable units, number them, and send them one by one. it is the responsibility of the transport layer at the receiving end to wait until all the different units belonging to the same process have arrived, check and pass those that are （5） free, and deliver them to the receiving process as a stream.

（2）是（）

A.connection

B.window

C.acknowledgement

D.destination
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