21. back up
　　1) 累積
　　The subway is running behind schedule. and traffic is backed up for blocks. I don’t know if we’ll make the 6:30 show.
　　2) 支持
　　I’ll back it up.
　　22. be bound for 到……地方
　　The bus is bound for New England.
　　23. be (feel) myself 找到自我
　　I’m feeling myself again.
　　24. be burned up 生氣
　　She was really burned up at the news.
　　25. be hard up for 缺少
　　I’m hard up for clothes, but I have a lot of books.
　　26. be head and shoulders above 好許多
　　In calculus. Joe is head and shoulders above his classmates.
　　27. be in the dark 蒙在鼓里
　　A: Do you have any idea what his notice is about?
　　B: I’m as in the dark as you are.
　　28. be stuck 卡住了
　　I can’t get this window open. It’s stuck.
　　29. bite off more than one can chew 貪多嚼不爛
　　A: I hear you’re taking an advanced physics course this semester.
　　B: I think I’ve bitten off more than I can chew.
　　30. break new ground 有了新的突破
　　His architectural design broke new ground in the field.