一、单项填空（本大题有20小题，每小题1分，共20分）
在每小题所给的四个选项中，选出可以填入空白处的最佳答案，并将答题卡上对应的选项涂黑。
1. Don’t you know it’s _____________ useful book for English learners?
A. a B. an C. the D. ／
2. Mr. Wang is a nice teacher. He often helps his students with _____________ physics after school.
A. them B. their C. theirs D. his
3. —If you help us solve the problems, you may go to America for a free trip.
—It’s a very nice _____________, but I really can’t accept it.
A. reason B. excuse C. offer D. decision
