下列哪一个不属于应用层协议？（）

A．用户数据协议UDF

B．文件传输协议FTP

C．域名服务DNS

D．电子邮件协议SMTF

—When are you going to fly to Taiwan this summer?

—________ we finish school. It will be a great holiday. [ ]

A. Because

B. Since

C. Until

D. As soon as