Our holidays were ruined by the werther ---have stayed at home

c 这里是虚拟语气的运用，表示与事实相反，我们不如待在家里，其实没有待在家，might引导虚拟句 as well 在这里是“到不如；还是...的好；最好...还是”的意思
翻译为：我们的假期被天气给破坏了，倒不如待在家里的好。
这里虚拟语气了，从句动作与主句动作同时发生（had + 过去分词）主句过去时were ruined 从句过去完成时 might have stayed

选C. might just as well是固定短语,译为:倒不如…还是...的好, 不妨\
might as well后可以接动词原形也可以接完成时,你把他看作是插入的就行了.
1 You might as well try.
你不妨试着用。
2 We might as well walk there.
我们不妨步行去。
3 You might just as well go as not.
你去也好，不去也好。
4 You might as well come with us.
他还是跟我们一起来的好。
5 I expect I might as well be going.
我看我也该走了。
6 We might as well get round the table.
我们不防围着桌子坐下。
7 Business is terrible I might as well pack up.
生意很不好做我还是关门算了.
8 As well might the glow_worm match himself against lightning.
这样，简直萤火虫也可以与闪电比光亮了。
9 Perhaps we might as well get round the table.
我们不妨围着桌子坐。