—Let’s go out for a walk.—You’d better not. These days the air is getting much ________ than it was before.
Your English book is on the floor.Please________.
Susan used to________to school,but she is used to________to school now.
I think every student should go to school________,but some of them are always late.
To keep safe,everyone________to wear a seat belt in the car.
The little boy can’t understand what you said.________,he is only two years old.