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3. (ア) ← [which](S) would be(V) useful(C) ….
※[ ]は主語になる単語が来る。主語になれるのは〇関係代名詞(アかエ)で、✕関係副詞
whose(関係代名詞:所有格)+名詞となるため、✕whose
4. (エ) ← ①Something took place. + ②My boss(S), usually very calm, could not have imagined(V) it(代名詞:目的格)(O) happening(C).
= Something took place which my boss, usually very calm, could not have imagined happening.
※it(代名詞:目的格)をwhich(関係代名詞:目的格)に変えて、①②の文を一つにまとめたのが問題文 something:whichの先行詞
参考にしてください。
ありがとうございます!