The well is too dangerous. We'd better _____ or it may hurt other people.[ ]

A. pull it down

B. pull down it

C. sit down

D. sit it down

我国下列科技成就出现的先后顺序是   

①“银河—I号”计算机研制成功          ②第一颗原子弹爆炸成功

③“东方红—号”人造地球卫星发射成功   ④“神舟五号”载人航天成功

A．①②③④

B．②③①④

C．④③①②

D．③①②④