下列选项中，关于合成高分子防水卷材外观质量要求，说法错误是（）。
A. 每卷折痕不超过2处，总长度不超过20mm
B. 杂质大于0.5mm颗粒不允许，每1m2不超过9mm2
C. 每卷胶块不超过6处，每处面积不大于4mm2
D. 每卷凹痕不超过6处，深度不超过本身厚度的40%
In which scenario do you use the Flashback Database feature？（）
A. when a table is dropped and you restore it from the Recycle Bin
B. when a user’s schema is dropped and you recover the user’s schema
C. when some incorrect data is inserted in the table and you retrieve the table to a state that was at a certain timestamp
D. when a row of a table is updated frequently and committed， and you view all the versions of the row within a specific duratio
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