TCP/IP (21) layer protocols provide services to the application (22) running on a computer. The application layer does not define the application itself, but rather it defines
(23) that applications need — like the ability to transfer a file in the case of HTTP. In short, the application layer provides an (24) 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 (25) layer consists of two main protocol options — the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).
A. application B. sessionC. physical D. transport
参考答案：D
解析：[参考译文]
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)。