情态动词的问题,我的问题是,两道题如果都是说明一件既定的事实的话,不表示推测,那么第一题应该选A,

情态动词的问题,我的问题是,两道题如果都是说明一件既定的事实的话,不表示推测,那么第一题应该选A,
第二题也应该选A,为什么说是选C的
1Why did Simon get home so late?-----He was so drunk that he _____his way back.
A.could not find B.could not have found
2 We have been waiting hours to see the famous pop singer ,but the airplane ______behind the timetable
A.must be B.will be C.must have been