If you had not got the first bus ,you --- late for meeting.

B would have been
虚拟语气
从句为过去完成时，主句应为过去将来完成时
这是规律

从 句 主 句
与现在事实相反 动词的过去式（be的过去式一般用were） would/ should/ could/ might + 动词原形
与过去事实相反 had + 过去分词 would/ should/ could/ might + have + 过去分词
与将来事实相反 动词过去式，should + 动词原形，were to + 动词原形 would/ should/ could/ might + 动词原形
注: 主句中的should只用于I、we，但在美国英语中，should常被would代替；从句中的should可用于各种人称。
l、表示与现在事实相反的假设和结果。如: If my brother were here, everything would be all right. 要是我哥哥在这儿 ，一切都没问题了。
2、表示与过去事实相反的假设和结果。如: If you had taken my advice，you wouldn't (couldn’t) have failed in the exam. 如果你按照我的建议去做，你一定不会(不可能)考试不及格。
3、表示与将来事实可能相反的假设和结果。如: If it were Sunday tomorrow, I should (would，could，might) go to see my grandmother. 如果明天是星期天，我就 (可能)去看望我奶 奶。If it were to snow this evening, they would not go out. 如果今晚下雪，他们将不出去了。

b
从句为过去完成时，主句应为过去将来完成时