问题
选择题
Only after he realized the significance of mastering two languages __________ English carefully.
A．did he set about learning
B．did he set out learning
C．had he set about learning
D．had he set out learning
答案
答案：A
考查倒装句,当only谓语句首时,主句部分采用部分倒装.句意,只有他认识到掌握两种语言的重要性后,他才开始认真学英语.set about doing sth开始做某事; set out出发;开始,后接不定式.故选A.