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（5）处应选择（）
A.application
B.session
C.physical
D.transport
参考答案：D
解析：
TCP/IP应用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可以传输文件。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应用层包括很多协议，HTTP仅是其中之一。TCP/IP传输层主要由两个可选的协议--传输控制协议（TCP）和用户数据报协议（UDP）组成。