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21. She wants to be a _________ (science) when she grows up.
22. There is a tall _________ (build) in front of the post office.
23. I can do my homework by _________ (I).
24. Today is Sunday. Let's go ________ (skate).
25. Please buy some _________ (toothbrush) for them.
(用所给词的适当形式填空) There is a tall _________ (build) in front of the post office.