21. The first meeting of________G-20 nations in April came at the height of the global financial crisis —    _________crisis that required unprecedented international cooperation to jumpstart the world's economies and help break the downward spiral that enveloped all our nations.

    A.  / ; the         B. the; a       C. a; the       D. the; the

22. --- Be careful! Don’t forget you are on a ladder.

   --- But you are holding it for me, nothing _________.

      A. to worry about    B worry about.  C. is worried about        D. worrying about