操作系统的主要功能是

A．对用户的数据文件进行管理，为用户管理文件提供方便

B．对计算机的所有资源进行统一控制和管理，为用户使用计算机提供方便

C．对源程序进行编译和运行

D．对汇编语言程序进行翻译

A celestial body will cross the prime vertical circle when the latitude is numerically（）.

A.greater than the declination and both are of the same name

B.less than the declination and both are of the same name

C.greater than the declination and both are of contrary name

D.less than the declination and both are of contrary name