数据库镜像可以用于( )。

A．保证数据库的完整性

B．实现数据库安全性

C．进行数据库恢复或并发操作

D．实现数据共享

It was _____ problem that _____ of the students could work it out [ ]

A. so hard a; a few

B. so hard a; few

C. such hard a; few

D. such a hard; a few