25. Only when I lost the opportunity to take my first job ____ how important it was to me.
A. I realized B. I had realized C, had 
I realized D . did I realize
Not once ________ to Michael that he could one day become a top student in his class.
| A．occurred it | B．it did occur | C．it occurred | D．did it occur |
— I don’t understand why you didn’t go to the lecture yesterday afternoon.
—I’m so sorry. But I _________ my homework.
| A．had done | B．was doing | C．would do | D．am doing |
The university estimates that living expenses for international students ________ around $8,450 a year,
which ________ a burden for some of them.
| A． | are；is | B． | are；are | C． | is；are | D． | is；is |
He ________ sleep, although he tried to, when he got on such a hunt for an idea until he had caught it.
| A．wouldn’t | B．shouldn’t | C．couldn’t | D．mustn’t |
warm at night, I would fill the woodstove, then set my alarm clock for midnight so I could
refill it.
| A． | Staying | B． | Stayed | C． | To stay | D． | Stay |