秦朝统治短暂而亡，其主要原因是[ ]
A．周边少数民族入侵
B．统治集团内讧变乱
C．宦官专权，外戚临朝
D．横征暴敛
答案：D
We were having lunch when the clock twelve.[ ]
A. beat
B. struck
C. hit
D. impacted
砂土是指粒径大于（）的颗粒含量超过全重的50%的土。
A．1mm
B．2mm
C．0.075mm
D．1.5mm
