A.had happened B.happened C.would happen D.has happened
       
        Explanation and Expansion
        A．参考答案 1）A 2）B 3） C
        B．评析
        谓语动词要求使用虚拟语气的状语从句一般是由as if/as though引导。该从句中时态的选择原则是：
        ① 如果从句中谓语动词与主句谓语动词同时进行，用过去进行时；
        ② 如果从句中谓语动词表示与主句谓语动词同时存在的状态，用一般过去时，如1）。
        ③ 如果从句中谓语动词表示先于主句谓语动词的动作或状态，用过去完成时，如2）。
        ④ 如果从句中谓语动词后于主句谓语动词发生，用would/might + 原形动词。
        Self-practice
        现在请试做《语法分册》159相关题目和162-166页综合测试题。
       
        三、定语从句
         定语从句在四级英语测试中占有一定的比重，大约在词汇语法题的7.4%，平均每次都有一题，实际测试中有时多达三道题，如1996年6月，98年6月和 2000年6月都有三倒考查定语从句的题。在我们讲四级英语测试对定语从句的考查之前，先让我们看以下几题：
       
        Test yourself
        1) Living in the central Australian desert has its problems, ____
        obtaining water is not the least.(1998.1)
        A.for which B.to which C.of which D.in which
       
        2) A survey was carried out on the death rate of new-born babies in that region, ____were surprising. (1999.1)
        A.as results B.which results C.the results of it D.the results of which
       
        3) We need a chairman ____.(1998.6)
        A.for whom everyone has confidence B.in whom everyone has confidence
        C.who everyone has confidence of D.whom everyone has confidence on
       
        4) The British are not so familiar with different cultures and other ways of doing things, ___ is often the case in other countries.(1998.6)