c.虚拟语气句,是对过去的虚拟,用if ...had done(been).,.would have done.
请相信我,这题必选c,因为他是说,如果我今天上班了,我肯定就注意到了,我今天上班是过去的事了,因而是对过去的虚拟.选c .选d是不可能的,那是对现在发生的事的虚拟,其固定搭配结构是：if .did/were(或者是should do,或者是were to do,共三种用法).,.would do .,本题后面是I would have noticed it,与d选项不搭配!