She_____by credit card but she preferred to use cash
问题描述：
She_____by credit card but she preferred to use cash
a.might pay b could have paid c.could pay d.must have paid
答
b could have paid
当然还是先看意思
她【本可以】用信用卡付款的,但她更喜欢使用现金（但他用现金付款了）
也就是属于虚拟语气的对过去的虚拟
d不对,她必定用了信用卡付款,但是她用了现金【前后矛盾】
c意思对,但是but she preferred 表明是事实,但是一定是发生过的事实,因为一般过去式啊
a表推测,和D表主观断定也是前后矛盾的,不是吗
==对过去的虚拟,多做就会有印象了,哈,多交流啊!