汉武帝派（）打击匈奴并修筑汉长城。

A．the different duties of police officers
B．crime in major metropolitan areas
C．civilian volunteer groups that help the police
D．how volunteer groups started

A．only a single classroom in the school
B．all the schools with classrooms
C．all the classrooms in the world
D．all the classrooms in a city or a country