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What do you think?
think単体に対しては,確かに疑問詞はwhatを使います。
what(何を)はthink(思う)の目的語だからです。
ところが,疑問詞+do you think (that)〜? の形の文で,疑問詞はthinkの目的語ではありません。
I think (that) he was absent from school "because he had a cold".
↓「下線部をたずねる疑問文」の要領で,
↓①"because 〜"をwhyに代えて文頭へ移動
↓②I thinkを疑問文の語順に整序
"Why" do you think he was absent from school?
〜その他の例〜
Who do you think I should invite?
Which do you think is better?
How do you think he solved the question?
When do you think these snowy days will end?
そういう事だったんですね!ありがとうございます!