对于易脱落的物品（木楔、盖板、混合料等），其高度必须低于矿车上沿（）mm。
A. 20
B. 30
C. 40
D. 50
Z是中国最大的城市，同时也是一座历史文化名城和著名的旅游城市。Z是（）
A. 上海
B. 重庆
C. 苏州
D. 北京
Which of the following statements best describes Flashback Versions Query？（）
A. Flashback Versions Query is used to make changes to multiple versions of data that existed between two points in time.
B. Flashback Versions Query is used to view all version changes on rows that existed between the time the query was executed and a point in time in the past.
C. Flashback Versions Query is used to view version changes and the SQL to undo those changes on rows that existed between two points in time.
D. Flashback Versions Query is used to view all version changes on rows that existed between two points in time.
机械开挖沟槽应预留一定厚度土层，由人工开挖至槽底设计高程，其厚度为（）。
A. 50～100mm
B. 100～150mm
C. 150～200mm
D. 200～300mm
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