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了：明白；指掌：指着手掌。形容对事物了解得非常清楚，象把东西放在手掌里给人家看一样。
[have sth.at one’s fingertips; know as one know his ten fingers; know like a book; know as well as a beggar knows his bag] 形容对情况极为清楚 对他们的活动,他完全了如指掌
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