隋朝开凿的大运河，其主要历史作用是：

A．方便隋炀帝南巡

B．巩固隋朝的统一

C．加强北部边防

D．促进南北经济交流

I am going to Qingdao and stay there for a week. _____ you are there, would you please

buy some books for me? [ ]

A. If

B. While

C. Since

D. As soon as