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All answers having "I don't think so!" as clue:

3 Character Answers:

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NAH

NOT



4 Character Answers:

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ASIF

UHUH



5 Character Answers:

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HUMPH

NOWAY



6 Character Answers:

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HARDLY



7 Character Answers:

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LIKEFUN



9 Character Answers:

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FATCHANCE

NOWAYJOSE



13 Character Answers:

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AREYOUSERIOUS

FORGETABOUTIT