题目

已知直线l1：ax－y＋2a＋1＝0和l2：2x－(a－1)y＋2＝0(a∈R)，则l1⊥l2的充要条件是a＝________. 答案： Nowadays it is difficult for people to find ______ work in the world because _________ economic situation is very serious.   A. /; an           B. /; the           C. the; the         D. a ; /