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21. School used to offer three after-class activities. 1. sports 2. art 3. volunteering，but this year, schools extra money may only can offer one activity. Which one do you choose and why?练习人数： 2
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22.练习人数： 14
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23.练习人数： 0
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24.练习人数： 2
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25.练习人数： 8
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26.练习人数： 6
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27.练习人数： 24
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28.练习人数： 12
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29.练习人数： 26
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30.练习人数： 10
