Tom and I see each other ________, but not as often as we used to.
Anyone can borrow books from this library _______ he keeps them clean and returns them in time.
So fast _____that it is difficult for us to imagine its speed.
Just as the tree is the unit of a forest and the book the unit of a library, ______ the molecule is the unit of matter.
The door opened and in ________.
______ nobody was very enthusiastic about it, they decided to cancel the trip.