________for the fact that he was working abroad, he would willingly have helped with the project

If it had been

If it had not been

Had it been

Hadn’t it beenĐáp án : B

If it had not been for + N hoặc If it had not been for the fact that + S + had past participle = nếu không vì cái gì đó trong quá khứ thì… -> câu điều kiện loại 3 giả định điều trái quá khứ