问题补充：
Sorry, I am too busy now. If I time, I would certainly go for an outing with you.
A. have had
B. had had
C. have
D. had
答案：
A. have had
B. had had
C. have
(答案→)D. had
解析：本题考查虚拟语气。句意：对不起，我很忙。要是我有空的话，我肯定会和你一起去郊游。既然I am too busy，说明“我有空”只能是对现在的一种假设，所以此处为虚拟语气。含有主从复合句的虚拟语气在if从句中谓语动词通常用过去式形式表现在，所以答案应选had。
