A。
虚拟预期在条件句中的应用：
1： If i were you , i would make better use of time at school.
如果我是你，我会更好地利用在学校的时间。

2：If you had taken your teacher's advice ,you wouldn't have failed in the exam .
如果你早听老师的话，考试就不会不及格。

3：If it should rain ,we would change our plan.
万一下雨的话，我们就改变计划。

用法
               条件从句的谓语动词             主句的谓语动词

与现在事实相反   动词过去式                     SHOULD+动词原形
                                                 WOULD+动词原形

与过去事实相反   HAD+过去分词                  SHOULD HAVE +过去分词
                                                 WOULD HAVE +过去分词

与将来事实可能   动词过去式                     SHOULD+动词原形
相反             SHOULD+动词原形               WOULD+动词原形  
                 WERE TO+动词原形  

注、：1在主句中SHOULD 用于第一人称，WOULD 用于任何人称。
      2主句中可用COULD，MIGHT代替SHOULD WOULD。
      3条件从句中一律用WERE，在口语或非正式语句中用WAS。但“IF I WERE YOU…… ”是固定说法