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. hardware B. software C. packetD. equipment

关于实验流行病学与观察流行病学的区别，以下说法正确的是（）

A.研究对象是随机样本

B.由研究者决定哪一组暴露于可疑危险因素，哪一组作为对照组

C.研究是前瞻性的

D.研究对象分为暴露组和非暴露组

E.暴露组和非暴露组的基本特征相似