用had to 或 must have been填空.
He didn't come to work yesterday.He ____ill.
He didn't come to the office this morning.He ______stay at home.
I don't think she was Austrian.She _____German.
I lost my pen so I _____buy a new one.
He forgot his case so he _______ return home.
She didn't hear the phone.She _____sleeping.
请说说他们有什么区别.