There aren’t many seats left for the concert; you had better make sure ____ two today.
The teacher is not only strict ____ his pupils but also strict ____ his own work.
Mary dropped in ____ Mr Smith, but he wasn' t at home, so she went to drop in ____ Mr Smith's office.
The worker can make chairs ____ wood, and also can make paper ____ wood.
The teacher is writing ____ a piece of chalk on the blackboard while the students are writing ____ink in exercise books.
—You know. Bob is a little slow ____ understanding, so…— So I have to be patient ______ him.