问题补充：
Sorry, I am too busy now. If I time, I would certainly go for an outing with you.
A. have
B. have had
C. had had
D. had
答案：
A. have
B. have had
C. had had
(答案→)D. had
解析：句意：对不起，我现在太忙了，如果我有时间，我肯定和你去郊游。本句中if引导的从句表示对现在的情况进行假设，从句用did，主句用would +do,故选D
