关系数据库系统中所管理的是（）。

A.一个mda文件

B.若干个mda文件

C.一个二维表

D.若干个二维表

We have received your letter of September 12，（）we are glad to know that you are interested in our electric heaters.

A.which

B.at which

C.from which

D.in which