历史课堂上，小明同学问老师：“北京人为什么过群居生活？”你能告诉他吗[ ]
A．生产工具简陋
B．血缘关系松散
C．自然条件艰难
D．体质发育不全
A
— Do you think it's going to rain tomorrow?
— _____________________[ ]
A. I think it
B. I don't think so
C. I not think it
SOM6板存储不能超过（）年，备件每年要上电一次。
