使用E-mail进行市场调研，以下说法错误的是（ ）。

以下不属于检索工具按照服务提供方式分类的是（ ）。

以下是对客户/服务器数据库系统的描述，请选择错误描述的选项。（ ）

Flow control is a function that prevents network congestion by ensuring that _（ ）_ do not overwhelm_（ ）_ with data．There are three commonly used methods for handling network congestion．_（ ）_is used by network devices to temporarily store bursts of excess data in memory until they can be processed．_（ ）_ are used by receiving devices to help prevent their buffers from overflowing．_（ ）_ is a flow－control scheme in which the source device requires an acknowledgment from the destination after a certain number of packets have been transmitted．

UNIX中，用来把一个进程的输出连接到另一个进程的输入的文件称为__（）__。