be flat的用法和樣例：
例句
- After running in the marathon, she was flat out for a week.
她參加馬拉松比賽后一星期都疲憊不堪。 - People used to think that the world was flat; now we know it is round.
從前人們以為地球是平的，現在知道是圓的。 - The land was flat and treeless and gave no cover to the troops.
該處地勢平坦又沒有樹木，部隊無處隱蔽。 - The market has been flat today.
今天市場上生意一直很清淡。
