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问题 填空题

下列化学符号中①2CO ②2Na+ ③Mg2+ ④⑤H2O数字“2”表示的意义是（均填序号）：

（1）离子个数是________；

（2）一个离子所带电荷数是________；

（3）分子个数是_______；

（4）元素的化合价数值是_______。

答案

②；③；①；④

单项选择题

In the open systems interconnection (OSI) reference model, "layer" means one of seven conceptually complete, (71) arranged groups of services, functions, and protocols, that extend across all open systems.

Application layer provides means for the application (72) to access the OSI environment. Presentation layer provides for the selection of a common syntax for representing data. (73) layer provides the means necessary for cooperating presentation entities to organize and synchronize their dialog and to manage their data exchange. Transport layer provides a reliable end-to-end data transfer service. Network layer provides for the entities in the transport layer the means for transferring blocks of data, by (74) and switching through the network between the open systems in which those entities reside. Data link layer provides services to transfer data between network layer entities. Physical layer provides the mechanical, electrical, functional, and procedural means to establish, maintain and (75) physical connections for transfer of bits over a transmission medium.

（72）处填（）。

A．procedures

B．processes

C．courses

D．schedules
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填空题

矿物的密度是指矿物单位体积的（），单位为（）。
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