Phrase "Don't you think"

[Lily]

Test No. incompl/elem-35 "Question Tags", question 5

I think you ought to go .........

(a) ought you?
(b) do I?
(c) don't you?
(d) do you?

Test No. incompl/elem-35 "Question Tags", answer 5

I think you ought to go don't you?

Correct answer: (c) don't you?

Your answer was: correct
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why is it not be "dont I"?

Lily

[Alan]

This means I think, dont YOU (think)?
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English as a Second Language
You can read my ESL story A day in the life of a policeman