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重点讲解：
1. come across 偶然发现；偶然遇见；
例句：I came across an old friend yesterday.
我昨天碰见一位老朋友。
2. write down 写下；记下；
例句：Only by writing things down could I bring some sort of order to the confusion.
只有把东西记下来，我才能从混乱中理出些头绪来。
3. cross off (从名单上)画掉，删掉 ；
例句：I checked the chart and found I had crossed off the wrong thing.
我检查了一下图表，结果发现我错删了东西。
4. once in a lifetime 终生难遇的；千载难逢的；
例句：It's a once in a lifetime chance.
这是一生难得的机会。
adj. 平的，与飞机有关的
n. 飞机，水平
