《礼记》所谓“天下为公，选贤任德”的社会局面，最早出现在

A．山顶洞人时期

B．半坡聚落时期

C．尧舜禹时期

D．黄帝和炎帝时期

The teacher wants his students _________  hard. [ ]

A. work

B. works

C. working

D. to work