21.Jumping out of ______ airplane at a height of 5000 kilometers is ______ unforgettable experience.
A. an , a B. an , an C. an , / D. a , an
22.Why ! Can you tell me ______ you want me to say .
A. what is it that B. what it is that C. how is it that D. how it is that
23._____ I am determined to do something , no one can persuade me to ______.
A. If , give away B. Although , give in C. As long as , give out D. Once , give up
