The plane had taken off. Paul realized he was on the wrong flight
Giải thích
Đáp án C.
Hai câu gốc: Máy bay cất cánh. Paul nhận ra anh ấy lên nhầm máy bay.
Câu C dùng đúng cấu trúc đảo ngữ với “Not until...:”
Not until/till + clause/ Adv of time + Auxiliary + S + V
Ex: I didn’t know that I had lost my key till I got home.
= Not until/till I got home did I know that I had lost my key.