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