按要求填空
⑴smoking(动词原形)
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⑵littering(动词原形)
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⑶too (同音词)
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⑷can(否定式)
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⑸shoudn't(完全形式)
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⑹they are(缩略式)
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⑺do (第三人称单数)
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⑻mean (第三人称单数)
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