TCP/IP (1) layer protocols provide services to the application (2)running on a computer. The application layer does not define the application itself, but rather it defines(3)that applications need - like the ability to transfer a file in the case of HTTP. In short, the application layer provides an (4)between software running on a computer and the network itself. The TCP/IP application layer includes a relatively large number of protocols,with HTTP being only one of those. The TCP/IP (5)layer consists of two main protocol options - the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).
空白（1）处应选择（）
A.application
B.transport
C.link
D.network
参考答案：A
解析：TCP/IP传输层协议为运行在计算机上的应用软件提供服务。应用层没有定义应用本身，而是定义了应用需要的服务，例如采用HTTP可以传输文本。简而言之，应用层提供了运行在计算机上的软件与网络之间的接口。TCP/IP应用层包括了很多的协议，HTTP仅是其中之一。TCP/IP传输层主要由两个可选的协议组成--传输控制协议（TCP）和用户数据报协议（UDP）。