All my friends like shopping in that supermarket. I wonder how long it _____. [ ]

A. has opened

B. has been open

C. has been opened

D. opened

Visual FoxPro关系数据库管理系统能实现的三种基本关系运算是( )。

A．索引，排序，查找

B．建库，录入，排序

C．选择，投影，联接

D．显示，统计，复制