Activities
Time
get up
6:00
eat breakfast
7:00
eat lunch
12:00
eat dinner
8:00
一物体放在凸透镜前18 cm处,在透镜另一侧20 cm处成一清晰像,则此透镜的焦距 ( )
下列城市属于贵州省且有“江南煤海”之称的是( )